diff --git a/json/escape_test.go b/json/escape_test.go
new file mode 100644
index 0000000..7b0a510
--- /dev/null
+++ b/json/escape_test.go
@@ -0,0 +1,371 @@
+package json
+
+import (
+	"bytes"
+	stdjson "encoding/json"
+	"fmt"
+	"math/bits"
+	"strings"
+	"testing"
+)
+
+func shouldEscape(b byte, escapeHTML bool) bool {
+	if b < 0x20 || b > 0x7f || b == '"' || b == '\\' {
+		return true
+	}
+	if escapeHTML && (b == '<' || b == '>' || b == '&') {
+		return true
+	}
+	return false
+}
+
+func expectedEscapeIndex(s string, escapeHTML bool) int {
+	for i := 0; i < len(s); i++ {
+		if shouldEscape(s[i], escapeHTML) {
+			return i
+		}
+	}
+	return -1
+}
+
+func TestEscapeIndexAllBytes(t *testing.T) {
+	for escapeHTML := range 2 {
+		htmlMode := escapeHTML == 1
+		for b := range 256 {
+			by := byte(b)
+			expected := shouldEscape(by, htmlMode)
+
+			// Test single-char string
+			s1 := string([]byte{by})
+			got1 := escapeIndex(s1, htmlMode)
+			if expected && got1 != 0 {
+				t.Errorf("byte 0x%02x (html=%v) single char: expected 0, got %d", by, htmlMode, got1)
+			} else if !expected && got1 != -1 {
+				t.Errorf("byte 0x%02x (html=%v) single char: expected -1, got %d", by, htmlMode, got1)
+			}
+
+			// Test 8-byte chunk with the byte at index 3
+			buf := []byte("aaaa.aaa")
+			buf[3] = by
+			s8 := string(buf)
+			got8 := escapeIndex(s8, htmlMode)
+			if expected && got8 != 3 {
+				t.Errorf("byte 0x%02x (html=%v) 8-byte chunk index 3: expected 3, got %d", by, htmlMode, got8)
+			} else if !expected && got8 != -1 {
+				t.Errorf("byte 0x%02x (html=%v) 8-byte chunk index 3: expected -1, got %d", by, htmlMode, got8)
+			}
+		}
+	}
+}
+
+func TestEscapeIndexChunkPositions(t *testing.T) {
+	specials := []byte{'\\', '"', '\n', '\r', '\t', '\x00', '\x1f', 0x80, 0xff}
+
+	for _, sp := range specials {
+		// Test escape character at every position in chunk 0 (0..7)
+		for pos := 0; pos < 8; pos++ {
+			b := []byte("01234567")
+			b[pos] = sp
+			s := string(b)
+			got := escapeIndex(s, false)
+			if got != pos {
+				t.Errorf("special 0x%02x in chunk 0 at pos %d: expected %d, got %d", sp, pos, pos, got)
+			}
+		}
+
+		// Test escape character at every position in chunk 1 (8..15)
+		for pos := 8; pos < 16; pos++ {
+			b := []byte("0123456789abcdef")
+			b[pos] = sp
+			s := string(b)
+			got := escapeIndex(s, false)
+			if got != pos {
+				t.Errorf("special 0x%02x in chunk 1 at pos %d: expected %d, got %d", sp, pos, pos, got)
+			}
+		}
+
+		// Test escape character at every position in chunk 2 (16..23)
+		for pos := 16; pos < 24; pos++ {
+			b := []byte("0123456789abcdef01234567")
+			b[pos] = sp
+			s := string(b)
+			got := escapeIndex(s, false)
+			if got != pos {
+				t.Errorf("special 0x%02x in chunk 2 at pos %d: expected %d, got %d", sp, pos, pos, got)
+			}
+		}
+
+		// Test escape character in tail (e.g. string length 11, pos 8, 9, 10)
+		for pos := 8; pos < 11; pos++ {
+			b := []byte("0123456789a")
+			b[pos] = sp
+			s := string(b)
+			got := escapeIndex(s, false)
+			if got != pos {
+				t.Errorf("special 0x%02x in tail at pos %d: expected %d, got %d", sp, pos, pos, got)
+			}
+		}
+	}
+}
+
+func TestEscapeIndexEdgeCases(t *testing.T) {
+	tests := []struct {
+		name       string
+		input      string
+		escapeHTML bool
+		expected   int
+	}{
+		{name: "empty", input: "", escapeHTML: false, expected: -1},
+		{name: "clean 7 bytes", input: "1234567", escapeHTML: false, expected: -1},
+		{name: "clean 8 bytes", input: "12345678", escapeHTML: false, expected: -1},
+		{name: "clean 16 bytes", input: "12345678abcdefgh", escapeHTML: false, expected: -1},
+		{name: "all spaces 8 bytes", input: "        ", escapeHTML: false, expected: -1},
+		{name: "all spaces 16 bytes", input: "                ", escapeHTML: false, expected: -1},
+		{name: "spaces then newline at index 7", input: "       \n", escapeHTML: false, expected: 7},
+		{name: "spaces then newline at index 15", input: "               \n", escapeHTML: false, expected: 15},
+		{name: "spaces then quote at index 7", input: "       \"", escapeHTML: false, expected: 7},
+		{name: "spaces then backslash at index 7", input: "       \\", escapeHTML: false, expected: 7},
+		{name: "newline then spaces", input: "\n       ", escapeHTML: false, expected: 0},
+		{name: "multiple escapes in chunk", input: "a\"b\\cdef", escapeHTML: false, expected: 1},
+		{name: "multiple escapes across chunks", input: "1234567\n12\"45678", escapeHTML: false, expected: 7},
+		{name: "html lt enabled", input: "abcdefg<", escapeHTML: true, expected: 7},
+		{name: "html lt disabled", input: "abcdefg<", escapeHTML: false, expected: -1},
+		{name: "html gt enabled", input: "12345678>abcdefg", escapeHTML: true, expected: 8},
+		{name: "html gt disabled", input: "12345678>abcdefg", escapeHTML: false, expected: -1},
+		{name: "html amp enabled", input: "12&45678", escapeHTML: true, expected: 2},
+		{name: "html amp disabled", input: "12&45678", escapeHTML: false, expected: -1},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := escapeIndex(tc.input, tc.escapeHTML)
+			if got != tc.expected {
+				t.Errorf("escapeIndex(%q, %v): expected %d, got %d", tc.input, tc.escapeHTML, tc.expected, got)
+			}
+		})
+	}
+}
+
+// simulateBigEndianChunkLayout converts a string so that each 8-byte chunk
+// has its bytes laid out in simulated big-endian word order when loaded
+// by a uint64 cast on a little-endian host.
+//
+// On a big-endian system, the byte at chunk offset k is stored at the most
+// significant byte (bits 63-8k..56-8k). On a little-endian host, uint64
+// loads place byte j of memory into bits 8j..8j+7.
+// Therefore, reversing the bytes within an 8-byte chunk causes a little-endian
+// uint64 load to produce the exact 64-bit integer that a big-endian host would
+// load from the original string.
+func simulateBigEndianWordLayout(s string) string {
+	b := []byte(s)
+	chunks := len(b) / 8
+	for i := 0; i < chunks; i++ {
+		start := i * 8
+		for j := 0; j < 4; j++ {
+			b[start+j], b[start+7-j] = b[start+7-j], b[start+j]
+		}
+	}
+	return string(b)
+}
+
+func TestEscapeIndexSimulatedEndianLayout(t *testing.T) {
+	origEndianness := endianness
+	defer func() {
+		endianness = origEndianness
+	}()
+
+	testCases := []struct {
+		name       string
+		original   string
+		escapeHTML bool
+	}{
+		{name: "clean single chunk", original: "abcdefgh", escapeHTML: false},
+		{name: "clean two chunks", original: "abcdefghijklmnop", escapeHTML: false},
+		{name: "clean with tail", original: "abcdefghijklmnopq", escapeHTML: false},
+		{name: "issue 170 backslash in chunk 0", original: `C:\Users\admin`, escapeHTML: false},
+		{name: "issue 170 tab", original: "key\tvalue", escapeHTML: false},
+		{name: "issue 170 newline at start", original: "\nfoo_bar_", escapeHTML: false},
+		{name: "issue 170 quote in chunk 0", original: `ab"cdefg`, escapeHTML: false},
+		{name: "backslash at chunk 0 pos 0", original: `\bcdefgh`, escapeHTML: false},
+		{name: "backslash at chunk 0 pos 1", original: `a\cdefgh`, escapeHTML: false},
+		{name: "backslash at chunk 0 pos 2", original: `ab\defgh`, escapeHTML: false},
+		{name: "backslash at chunk 0 pos 3", original: `abc\efgh`, escapeHTML: false},
+		{name: "backslash at chunk 0 pos 4", original: `abcd\fgh`, escapeHTML: false},
+		{name: "backslash at chunk 0 pos 5", original: `abcde\gh`, escapeHTML: false},
+		{name: "backslash at chunk 0 pos 6", original: `abcdef\h`, escapeHTML: false},
+		{name: "backslash at chunk 0 pos 7", original: `abcdefg\`, escapeHTML: false},
+		{name: "backslash at chunk 1 pos 0", original: `abcdefgh\jklmnop`, escapeHTML: false},
+		{name: "backslash at chunk 1 pos 3", original: `abcdefghijk\mnop`, escapeHTML: false},
+		{name: "backslash at chunk 1 pos 7", original: `abcdefghijklmno\`, escapeHTML: false},
+		{name: "spaces then newline", original: "       \n", escapeHTML: false},
+		{name: "spaces then newline chunk 1", original: "        " + "       \n", escapeHTML: false},
+		{name: "control char at start", original: "\x01bcdefgh", escapeHTML: false},
+		{name: "html tags enabled", original: "abc<efgh", escapeHTML: true},
+		{name: "html tags disabled", original: "abc<efgh", escapeHTML: false},
+		{name: "html amp enabled", original: "abcdefg&", escapeHTML: true},
+		{name: "html amp disabled", original: "abcdefg&", escapeHTML: false},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			expected := expectedEscapeIndex(tc.original, tc.escapeHTML)
+
+			// 1. Verify under native Little-Endian mode (endianness = 0)
+			endianness = 0
+			gotLE := escapeIndex(tc.original, tc.escapeHTML)
+			if gotLE != expected {
+				t.Fatalf("LE mode: escapeIndex(%q, %v): expected %d, got %d", tc.original, tc.escapeHTML, expected, gotLE)
+			}
+
+			// 2. Verify under simulated Big-Endian mode (endianness = 1)
+			// We feed the simulated big-endian layout into escapeIndex.
+			// The simulated string has its 8-byte words reversed so that on this LE host,
+			// uint64 memory loads match the big-endian register layout.
+			endianness = 1
+			simulatedBE := simulateBigEndianWordLayout(tc.original)
+			gotBE := escapeIndex(simulatedBE, tc.escapeHTML)
+			if gotBE != expected {
+				t.Fatalf("BE mode: escapeIndex(simulated %q, %v): expected %d, got %d", tc.original, tc.escapeHTML, expected, gotBE)
+			}
+		})
+	}
+}
+
+func TestEscapeIndexWordBitPatterns(t *testing.T) {
+	// Directly validate the bit arithmetic on 64-bit words for both byte orders.
+	// For every byte position k in 0..7 and various special characters:
+	specials := []byte{'\\', '"', '\n', '\r', '\t', '<', '>', '&', 0x1f, 0x80}
+
+	for _, sp := range specials {
+		for k := 0; k < 8; k++ {
+			// Construct an 8-byte chunk with safe ASCII 'A' and the special byte at index k.
+			var b [8]byte
+			for i := range b {
+				b[i] = 'A'
+			}
+			b[k] = sp
+
+			// Little-endian uint64 value: byte k is at bits 8k..8k+7
+			var nLE uint64
+			for i := 0; i < 8; i++ {
+				nLE |= uint64(b[i]) << (8 * i)
+			}
+
+			// Big-endian uint64 value: byte k is at bits (7-k)*8..(7-k)*8+7
+			var nBE uint64
+			for i := 0; i < 8; i++ {
+				nBE |= uint64(b[i]) << (8 * (7 - i))
+			}
+
+			// Verify that reversing the BE word restores the LE word
+			if bits.ReverseBytes64(nBE) != nLE {
+				t.Fatalf("ReverseBytes64(nBE) != nLE for special 0x%02x at pos %d", sp, k)
+			}
+
+			// Verify that below/contains mask on reversed word yields exact index k
+			mask := nLE | below(nLE, 0x20) | contains(nLE, '"') | contains(nLE, '\\')
+			mask |= contains(nLE, '<') | contains(nLE, '>') | contains(nLE, '&')
+			if (mask & msb) == 0 {
+				t.Fatalf("expected mask MSB to be set for special 0x%02x at pos %d", sp, k)
+			}
+			idx := bits.TrailingZeros64(mask&msb) / 8
+			if idx != k {
+				t.Fatalf("expected index %d for special 0x%02x, got %d", k, sp, idx)
+			}
+		}
+	}
+}
+
+func TestEscapeIndexIssue170Reproducer(t *testing.T) {
+	cases := []string{
+		`C:\Users\admin`, // backslash near the start
+		"key\tvalue",     // tab
+		"\nfoo_bar_",     // newline at the very start
+		`ab"cdefg`,       // quote near the start
+		`"quoted"`,
+		`C:\Program Files\App\bin`,
+		"multi\nline\tstring\"with\\escapes",
+	}
+
+	for _, in := range cases {
+		type W struct {
+			V string `json:"v"`
+		}
+		var buf bytes.Buffer
+		enc := NewEncoder(&buf)
+		enc.SetAppendNewline(false)
+		if err := enc.Encode(W{V: in}); err != nil {
+			t.Errorf("%q: encode error: %v", in, err)
+			continue
+		}
+
+		var got W
+		if err := stdjson.Unmarshal(buf.Bytes(), &got); err != nil {
+			t.Errorf("%q: got invalid JSON: %v\n  raw: %s", in, err, buf.Bytes())
+			continue
+		}
+		if got.V != in {
+			t.Errorf("%q: came back as %q", in, got.V)
+		}
+	}
+}
+
+func TestStringEncodingRoundTrip(t *testing.T) {
+	// Exhaustively test string encoding against standard library encoding/json
+	testStrings := []string{
+		"",
+		"a",
+		"hello world",
+		`"`,
+		`\`,
+		`""`,
+		`\\`,
+		`"hello"`,
+		`c:\path\to\file`,
+		"\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f",
+		"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f",
+		"        \n",
+		"       \n ",
+		"      \n  ",
+		"     \n   ",
+		"    \n    ",
+		"   \n     ",
+		"  \n      ",
+		" \n       ",
+		"\n        ",
+		strings.Repeat("a", 7) + "\"",
+		strings.Repeat("a", 8) + "\"",
+		strings.Repeat("a", 9) + "\"",
+		strings.Repeat("a", 15) + "\\",
+		strings.Repeat("a", 16) + "\\",
+		strings.Repeat("a", 17) + "\\",
+		"<script>alert('xss')&;</script>",
+		"Unicode: \u0000 \u001f \u0020 \u007f \u0080 \u00ff \u4e16\u754c",
+	}
+
+	for i, s := range testStrings {
+		t.Run(fmt.Sprintf("case-%d", i), func(t *testing.T) {
+			gotJSON, err := Marshal(s)
+			if err != nil {
+				t.Fatalf("Marshal(%q) error: %v", s, err)
+			}
+
+			wantJSON, err := stdjson.Marshal(s)
+			if err != nil {
+				t.Fatalf("stdjson.Marshal(%q) error: %v", s, err)
+			}
+
+			if !bytes.Equal(gotJSON, wantJSON) {
+				t.Errorf("Marshal mismatch for %q:\n got:  %s\n want: %s", s, gotJSON, wantJSON)
+			}
+
+			var unmarshaled string
+			if err := Unmarshal(gotJSON, &unmarshaled); err != nil {
+				t.Fatalf("Unmarshal failed: %v", err)
+			}
+			if unmarshaled != s {
+				t.Errorf("roundtrip mismatch: got %q, want %q", unmarshaled, s)
+			}
+		})
+	}
+}
diff --git a/json/string.go b/json/string.go
index a9a972b..29f0efb 100644
--- a/json/string.go
+++ b/json/string.go
@@ -17,7 +17,10 @@ const (
 // escaping, the return value is -1.
 func escapeIndex(s string, escapeHTML bool) int {
 	chunks := stringToUint64(s)
-	for _, n := range chunks {
+	for i, n := range chunks {
+		if endianness == 1 {
+			n = bits.ReverseBytes64(n)
+		}
 		// combine masks before checking for the MSB of each byte. We include
 		// `n` in the mask to check whether any of the *input* byte MSBs were
 		// set (i.e. the byte was outside the ASCII range).
@@ -26,7 +29,7 @@ func escapeIndex(s string, escapeHTML bool) int {
 			mask |= contains(n, '<') | contains(n, '>') | contains(n, '&')
 		}
 		if (mask & msb) != 0 {
-			return bits.TrailingZeros64(mask&msb) / 8
+			return i*8 + bits.TrailingZeros64(mask&msb)/8
 		}
 	}