diff --git a/Makefile b/Makefile
index b9b338b3..0c1ba328 100644
--- a/Makefile
+++ b/Makefile
@@ -206,6 +206,9 @@ test-records: chibi-scheme$(EXE)
 test-weak: chibi-scheme$(EXE)
 	LD_LIBRARY_PATH=".:$(LD_LIBRARY_PATH)" ./chibi-scheme$(EXE) tests/weak-tests.scm
 
+test-unicode: chibi-scheme$(EXE)
+	LD_LIBRARY_PATH=".:$(LD_LIBRARY_PATH)" ./chibi-scheme$(EXE) tests/unicode-tests.scm
+
 test-libs: chibi-scheme$(EXE)
 	LD_LIBRARY_PATH=".:$(LD_LIBRARY_PATH)" ./chibi-scheme$(EXE) tests/lib-tests.scm
 
diff --git a/eval.c b/eval.c
index ede8e5bd..4683edea 100644
--- a/eval.c
+++ b/eval.c
@@ -1142,13 +1142,13 @@ static sexp sexp_string_cmp_op (sexp ctx sexp_api_params(self, n), sexp str1, se
 
 #if SEXP_USE_UTF8_STRINGS
 
-static int sexp_utf8_initial_byte_count(int c) {
+static int sexp_utf8_initial_byte_count (int c) {
   if (c < 0xC0) return 1;
   if (c < 0xE0) return 2;
   return ((c>>4)&1)+3;
 }
 
-static int sexp_utf8_char_byte_count(int c) {
+static int sexp_utf8_char_byte_count (int c) {
   if (c < 0x80) return 1;
   if (c < 0x800) return 2;
   if (c < 0x10000) return 3;
@@ -1163,20 +1163,6 @@ static int sexp_string_utf8_length (unsigned char *p, int len) {
   return i;
 }
 
-sexp sexp_string_index_to_offset (sexp ctx sexp_api_params(self, n), sexp str, sexp index) {
-  sexp_sint_t i, j, limit;
-  unsigned char *p;
-  sexp_assert_type(ctx, sexp_stringp, SEXP_STRING, str);
-  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, index);
-  p = (unsigned char*)sexp_string_data(str);
-  limit = sexp_string_length(str);
-  for (j=0, i=sexp_unbox_fixnum(index); i>0 && j<limit; i--)
-    j += sexp_utf8_initial_byte_count(p[j]);
-  if (i>0)
-    return sexp_user_exception(ctx, self, "string-index->offset: index out of range", index);
-  return sexp_make_fixnum(j);
-}
-
 sexp sexp_string_utf8_ref (sexp ctx, sexp str, sexp i) {
   unsigned char *p=(unsigned char*)sexp_string_data(str) + sexp_unbox_fixnum(i);
   if (*p < 0x80)
@@ -1200,17 +1186,6 @@ sexp sexp_string_utf8_index_ref (sexp ctx sexp_api_params(self, n), sexp str, se
   return sexp_string_utf8_ref(ctx, str, off);
 }
 
-void sexp_utf8_encode_char (unsigned char* p, int len, int c) {
-  switch (len) {
-  case 4:  *p++ = (0xF0 + ((c)>>18)); *p++ = (0x80 + ((c>>12)&0x3F));
-    *p++ = (0x80 + ((c>>6)&0x3F));    *p = (0x80 + (c&0x3F)); break;
-  case 3:  *p++ = (0xE0 + ((c)>>12)); *p++ = (0x80 + ((c>>6)&0x3F));
-    *p = (0x80 + (c&0x3F)); break;
-  case 2:  *p++ = (0xC0 + ((c)>>6));  *p = (0x80 + (c&0x3F)); break;
-  default: *p = c; break;
-  }
-}
-
 void sexp_write_utf8_char (sexp ctx, int c, sexp out) {
   unsigned char buf[8];
   int len = sexp_utf8_char_byte_count(c);
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 700f011b..cd2d4727 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -1034,6 +1034,12 @@ SEXP_API sexp sexp_range_exception (sexp ctx, sexp obj, sexp start, sexp end);
 SEXP_API sexp sexp_print_exception_op (sexp ctx sexp_api_params(self, n), sexp exn, sexp out);
 SEXP_API void sexp_init(void);
 
+#if SEXP_USE_UTF8_STRINGS
+SEXP_API sexp sexp_string_index_to_offset (sexp ctx sexp_api_params(self, n), sexp str, sexp index);
+SEXP_API sexp sexp_utf8_substring_op (sexp ctx sexp_api_params(self, n), sexp str, sexp start, sexp end);
+SEXP_API void sexp_utf8_encode_char (unsigned char* p, int len, int c);
+#endif
+
 #define sexp_assert_type(ctx, pred, type_id, obj) if (! pred(obj)) return sexp_type_exception(ctx, self, type_id, obj)
 
 #define SEXP_COPY_DEFAULT SEXP_ZERO
diff --git a/opcodes.c b/opcodes.c
index e7a13eee..9bfdeed7 100644
--- a/opcodes.c
+++ b/opcodes.c
@@ -10,6 +10,7 @@
 #define _FN2OPT(rt, a1, a2, s, d, f) _FN(SEXP_OP_FCALL2, 1, 1, rt, a1, a2, SEXP_FALSE, s, d, f)
 #define _FN2OPTP(rt, a1, a2, s, d, f) _FN(SEXP_OP_FCALL2, 1, 3, rt, a1, a2, SEXP_FALSE, s, d, f)
 #define _FN3(rt, a1, a2, a3, s, d, f) _FN(SEXP_OP_FCALL3, 3, 0, rt, a1, a2, a3, s, d, f)
+#define _FN3OPT(rt, a1, a2, a3, s, d, f) _FN(SEXP_OP_FCALL3, 2, 1, rt, a1, a2, a3, s, d, f)
 #define _FN4(rt, a1, a2, a3, s, d, f) _FN(SEXP_OP_FCALL4, 4, 0, rt, a1, a2, a3, s, d, f)
 #define _PARAM(n, t) _OP(SEXP_OPC_PARAMETER, SEXP_OP_PARAMETER_REF, 0, 1, t, t, SEXP_FALSE, SEXP_FALSE, 0, n, SEXP_FALSE, 0)
 
@@ -120,7 +121,6 @@ _FN2OPT(_I(SEXP_STRING), _I(SEXP_FIXNUM), _I(SEXP_CHAR), "make-string", sexp_mak
 _FN2OPT(_I(SEXP_STRING), _I(SEXP_FIXNUM), _I(SEXP_FIXNUM), "make-byte-vector", SEXP_ZERO, sexp_make_bytes_op),
 _FN2OPT(_I(SEXP_NUMBER), _I(SEXP_STRING), _I(SEXP_FIXNUM), "string->number", SEXP_TEN, sexp_string_to_number_op),
 _FN3(_I(SEXP_FIXNUM), _I(SEXP_STRING), _I(SEXP_STRING), _I(SEXP_BOOLEAN), "string-cmp", 0, sexp_string_cmp_op),
-_FN3(_I(SEXP_STRING), _I(SEXP_STRING), _I(SEXP_FIXNUM), _I(SEXP_FIXNUM), "substring", 0, sexp_substring_op),
 _FN1(_I(SEXP_SYMBOL), _I(SEXP_STRING), "string->symbol", 0, sexp_string_to_symbol_op),
 _FN2OPT(_I(SEXP_STRING), SEXP_NULL, _I(SEXP_STRING), "string-concatenate", SEXP_FALSE, sexp_string_concatenate_op),
 _FN2(_I(SEXP_OBJECT), _I(SEXP_OBJECT), SEXP_NULL, "memq", 0, sexp_memq_op),
@@ -150,6 +150,10 @@ _FN2(_I(SEXP_NUMBER), _I(SEXP_NUMBER), _I(SEXP_NUMBER), "expt", 0, sexp_expt_op)
 _FN2(_I(SEXP_FIXNUM), _I(SEXP_STRING), _I(SEXP_FIXNUM), "string-index->offset", 0, sexp_string_index_to_offset),
 _FN2(_I(SEXP_CHAR), _I(SEXP_STRING), _I(SEXP_FIXNUM), "string-ref", 0, sexp_string_utf8_index_ref),
 _FN3(SEXP_VOID, _I(SEXP_STRING), _I(SEXP_FIXNUM), _I(SEXP_CHAR), "string-set!", 0, sexp_string_utf8_index_set),
+_FN3OPT(_I(SEXP_STRING), _I(SEXP_STRING), _I(SEXP_FIXNUM), _I(SEXP_FIXNUM), "substring-cursor", SEXP_FALSE, sexp_substring_op),
+_FN3OPT(_I(SEXP_STRING), _I(SEXP_STRING), _I(SEXP_FIXNUM), _I(SEXP_FIXNUM), "substring", SEXP_FALSE, sexp_utf8_substring_op),
+#else
+_FN3OPT(_I(SEXP_STRING), _I(SEXP_STRING), _I(SEXP_FIXNUM), _I(SEXP_FIXNUM), "substring", SEXP_FALSE, sexp_substring_op),
 #endif
 #if SEXP_USE_TYPE_DEFS
 _FN3(_I(SEXP_TYPE), _I(SEXP_STRING), _I(SEXP_TYPE), SEXP_NULL, "register-simple-type", 0, sexp_register_simple_type_op),
diff --git a/sexp.c b/sexp.c
index 0d79e42c..68c35bf9 100644
--- a/sexp.c
+++ b/sexp.c
@@ -720,10 +720,64 @@ sexp sexp_make_bytes_op (sexp ctx sexp_api_params(self, n), sexp len, sexp i) {
   return s;
 }
 
+#if SEXP_USE_UTF8_STRINGS
+
+static int sexp_utf8_initial_byte_count (int c) {
+  if (c < 0xC0) return 1;
+  if (c < 0xE0) return 2;
+  return ((c>>4)&1)+3;
+}
+
+static int sexp_utf8_char_byte_count (int c) {
+  if (c < 0x80) return 1;
+  if (c < 0x800) return 2;
+  if (c < 0x10000) return 3;
+  return 4;
+}
+
+void sexp_utf8_encode_char (unsigned char* p, int len, int c) {
+  switch (len) {
+  case 4:  *p++ = (0xF0 + ((c)>>18)); *p++ = (0x80 + ((c>>12)&0x3F));
+    *p++ = (0x80 + ((c>>6)&0x3F));    *p = (0x80 + (c&0x3F)); break;
+  case 3:  *p++ = (0xE0 + ((c)>>12)); *p++ = (0x80 + ((c>>6)&0x3F));
+    *p = (0x80 + (c&0x3F)); break;
+  case 2:  *p++ = (0xC0 + ((c)>>6));  *p = (0x80 + (c&0x3F)); break;
+  default: *p = c; break;
+  }
+}
+
+sexp sexp_string_index_to_offset (sexp ctx sexp_api_params(self, n), sexp str, sexp index) {
+  sexp_sint_t i, j, limit;
+  unsigned char *p;
+  sexp_assert_type(ctx, sexp_stringp, SEXP_STRING, str);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, index);
+  p = (unsigned char*)sexp_string_data(str);
+  limit = sexp_string_length(str);
+  for (j=0, i=sexp_unbox_fixnum(index); i>0 && j<limit; i--)
+    j += sexp_utf8_initial_byte_count(p[j]);
+  if (i>0)
+    return sexp_user_exception(ctx, self, "string-index->offset: index out of range", index);
+  return sexp_make_fixnum(j);
+}
+
+#endif
+
 sexp sexp_make_string_op (sexp ctx sexp_api_params(self, n), sexp len, sexp ch)
 {
   sexp i = (sexp_charp(ch) ? sexp_make_fixnum(sexp_unbox_character(ch)) : ch);
   sexp_gc_var2(b, s);
+#if SEXP_USE_UTF8_STRINGS
+  int j, clen;
+  if (sexp_charp(ch) && (sexp_unbox_character(ch) >= 0x80)) {
+    sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, len);
+    clen = sexp_utf8_char_byte_count(sexp_unbox_character(ch));
+    b = sexp_make_bytes_op(ctx sexp_api_pass(self, n),
+                           sexp_fx_mul(len, sexp_make_fixnum(clen)), SEXP_VOID);
+    for (j=0; j<sexp_unbox_fixnum(len); j++)
+      sexp_utf8_encode_char((unsigned char*)sexp_bytes_data(b)+(j*clen), clen,
+                            sexp_unbox_character(ch));
+  } else
+#endif
   b = sexp_make_bytes_op(ctx sexp_api_pass(self, n), len, i);
   if (sexp_exceptionp(b)) return b;
 #if SEXP_USE_PACKED_STRINGS
@@ -734,7 +788,7 @@ sexp sexp_make_string_op (sexp ctx sexp_api_params(self, n), sexp len, sexp ch)
   s = sexp_alloc_type(ctx, string, SEXP_STRING);
   sexp_string_bytes(s) = b;
   sexp_string_offset(s) = 0;
-  sexp_string_length(s) = sexp_unbox_fixnum(len);
+  sexp_string_length(s) = sexp_bytes_length(b);
   sexp_gc_release2(ctx);
   return s;
 #endif
@@ -769,6 +823,17 @@ sexp sexp_substring_op (sexp ctx sexp_api_params(self, n), sexp str, sexp start,
   return res;
 }
 
+#if SEXP_USE_UTF8_STRINGS
+sexp sexp_utf8_substring_op (sexp ctx sexp_api_params(self, n), sexp str, sexp start, sexp end) {
+  sexp_assert_type(ctx, sexp_stringp, SEXP_STRING, str);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, start);
+  start = sexp_string_index_to_offset(ctx sexp_api_pass(self, n), str, start);
+  if (sexp_fixnump(end))
+    end = sexp_string_index_to_offset(ctx sexp_api_pass(self, n), str, end);
+  return sexp_substring_op(ctx sexp_api_pass(self, n), str, start, end);
+}
+#endif
+
 sexp sexp_string_concatenate_op (sexp ctx sexp_api_params(self, n), sexp str_ls, sexp sep) {
   sexp res, ls;
   sexp_uint_t len=0, i=0, sep_len=0;
@@ -1540,23 +1605,18 @@ sexp sexp_read_number (sexp ctx, sexp in, int base) {
 }
 
 #if SEXP_USE_UTF8_STRINGS
-static int sexp_decode_utf8_char(const unsigned char* str) {
-  const unsigned char* tail=str;
-  int i = str[0], res = -1;
-  if ((i >= 0xC0) && (i <= 0xF7)) {
-    if (i < 0xE0) {
-      res = ((i&0x3F)<<6) + (str[1]&0x3F);
-      tail = str + 2;
-    } else if (i < 0xF0) {
-      res = ((i&0x1F)<<12) + ((str[1]&0x3F)<<6) + (str[2]&0x3F);
-      tail = str + 3;
-    } else {
-      res = ((i&0x0F)<<16) + ((str[1]&0x3F)<<6)
-        + ((str[2]&0x3F)<<6) + (str[3]&0x3F);
-      tail = str + 4;
+static int sexp_decode_utf8_char(const unsigned char* s) {
+  int i = s[0], len = strlen((const char*)s);
+  if ((i >= 0xC0) && (i <= 0xF7) && (s[1]>>6 == 2)) {
+    if ((i < 0xE0) && (len == 2)) {
+      return ((i&0x3F)<<6) + (s[1]&0x3F);
+    } else if ((i < 0xF0) && (len == 3) && (s[2]>>6 == 2)) {
+      return ((i&0x1F)<<12) + ((s[1]&0x3F)<<6) + (s[2]&0x3F);
+    } else if ((len == 4) && (s[2]>>6 == 2) && (s[3]>>6 == 2)) {
+      return ((i&0x0F)<<16) + ((s[1]&0x3F)<<6) + ((s[2]&0x3F)<<6) + (s[3]&0x3F);
     }
   }
-  return *tail ? -1 : res;
+  return -1;
 }
 #endif
 
diff --git a/tests/unicode-tests.scm b/tests/unicode-tests.scm
new file mode 100644
index 00000000..faec75e7
--- /dev/null
+++ b/tests/unicode-tests.scm
@@ -0,0 +1,43 @@
+;; These tests are only valid if chibi-scheme is compiled with Unicode
+;; support (SEXP_USE_UTF8_STRINGS).
+
+(import (chibi test))
+
+(test-begin "unicode")
+
+(test #\Р (string-ref "Русский" 0))
+(test #\и (string-ref "Русский" 5))
+(test #\й (string-ref "Русский" 6))
+
+(test 7 (string-length "Русский"))
+
+(test #\二 (string-ref "二本語" 0))
+(test #\本 (string-ref "二本語" 1))
+(test #\語 (string-ref "二本語" 2))
+
+(test 3 (string-length "二本語"))
+
+(test '(#\二 #\本 #\語) (string->list "二本語"))
+(test "二本語" (list->string '(#\二 #\本 #\語)))
+
+(test "二本" (substring "二本語" 0 2))
+(test "本語" (substring "二本語" 1 3))
+
+(test "二-語"
+      (let ((s (substring "二本語" 0 3)))
+        (string-set! s 1 #\-)
+        s))
+
+(test "二本人"
+      (let ((s (substring "二本語" 0 3)))
+        (string-set! s 2 #\人)
+        s))
+
+(test "字字字" (make-string 3 #\字))
+
+(test "字字字"
+      (let ((s (make-string 3)))
+        (string-fill! s #\字)
+        s))
+
+(test-end)