--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,9 @@
 2010-01-09  Bruno Haible  <bruno@clisp.org>
 
+	Tests for module 'unistr/u16-to-u8'.
+	* modules/unistr/u16-to-u8-tests: New file.
+	* tests/unistr/test-u16-to-u8.c: New file.
+
 	Tests for module 'unistr/u8-to-u32'.
 	* modules/unistr/u8-to-u32-tests: New file.
 	* tests/unistr/test-u8-to-u32.c: New file.

new file mode 100644
--- /dev/null
+++ b/modules/unistr/u16-to-u8-tests
@@ -0,0 +1,13 @@
+Files:
+tests/unistr/test-u16-to-u8.c
+tests/macros.h
+
+Depends-on:
+unistr/u8-cmp
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-u16-to-u8
+check_PROGRAMS += test-u16-to-u8
+test_u16_to_u8_SOURCES = unistr/test-u16-to-u8.c

new file mode 100644
--- /dev/null
+++ b/tests/unistr/test-u16-to-u8.c
@@ -0,0 +1,159 @@
+/* Test of u16_to_u8() function.
+   Copyright (C) 2010 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2010.  */
+
+#include <config.h>
+
+#include "unistr.h"
+
+#include <errno.h>
+
+#include "macros.h"
+
+static int
+check (const uint16_t *input, size_t input_length,
+       const uint8_t *expected, size_t expected_length)
+{
+  size_t length;
+  uint8_t *result;
+
+  /* Test return conventions with resultbuf == NULL.  */
+  result = u16_to_u8 (input, input_length, NULL, &length);
+  if (!(result != NULL))
+    return 1;
+  if (!(length == expected_length))
+    return 2;
+  if (!(u8_cmp (result, expected, expected_length) == 0))
+    return 3;
+  free (result);
+
+  /* Test return conventions with resultbuf too small.  */
+  if (expected_length > 0)
+    {
+      uint8_t *preallocated;
+
+      length = expected_length - 1;
+      preallocated = (uint8_t *) malloc (length * sizeof (uint8_t));
+      result = u16_to_u8 (input, input_length, preallocated, &length);
+      if (!(result != NULL))
+        return 4;
+      if (!(result != preallocated))
+        return 5;
+      if (!(length == expected_length))
+        return 6;
+      if (!(u8_cmp (result, expected, expected_length) == 0))
+        return 7;
+      free (result);
+      free (preallocated);
+    }
+
+  /* Test return conventions with resultbuf large enough.  */
+  {
+    uint8_t *preallocated;
+
+    length = expected_length;
+    preallocated = (uint8_t *) malloc (length * sizeof (uint8_t));
+    result = u16_to_u8 (input, input_length, preallocated, &length);
+    if (!(result != NULL))
+      return 8;
+    if (!(preallocated == NULL || result == preallocated))
+      return 9;
+    if (!(length == expected_length))
+      return 10;
+    if (!(u8_cmp (result, expected, expected_length) == 0))
+      return 11;
+    free (preallocated);
+  }
+
+  return 0;
+}
+
+int
+main ()
+{
+  /* Empty string.  */
+  ASSERT (check (NULL, 0, NULL, 0) == 0);
+
+  /* Simple string.  */
+  { /* "Grüß Gott. Здравствуйте! x=(-b±sqrt(b²-4ac))/(2a)  日本語,中文,한글" */
+    static const uint16_t input[] =
+      { 'G', 'r', 0x00FC, 0x00DF, ' ', 'G', 'o', 't', 't', '.', ' ',
+        0x0417, 0x0434, 0x0440, 0x0430, 0x0432, 0x0441, 0x0442, 0x0432, 0x0443,
+        0x0439, 0x0442, 0x0435, '!', ' ',
+        'x', '=', '(', '-', 'b', 0x00B1, 's', 'q', 'r', 't', '(', 'b', 0x00B2,
+        '-', '4', 'a', 'c', ')', ')', '/', '(', '2', 'a', ')', ' ', ' ',
+        0x65E5, 0x672C, 0x8A9E, ',', 0x4E2D, 0x6587, ',', 0xD55C, 0xAE00, '\n'
+      };
+    static const uint8_t expected[] =
+      { 'G', 'r', 0xC3, 0xBC, 0xC3, 0x9F, ' ', 'G', 'o', 't', 't', '.', ' ',
+        0xD0, 0x97, 0xD0, 0xB4, 0xD1, 0x80, 0xD0, 0xB0, 0xD0, 0xB2, 0xD1, 0x81,
+        0xD1, 0x82, 0xD0, 0xB2, 0xD1, 0x83, 0xD0, 0xB9, 0xD1, 0x82, 0xD0, 0xB5,
+        '!', ' ', 'x', '=', '(', '-', 'b', 0xC2, 0xB1, 's', 'q', 'r', 't', '(',
+        'b', 0xC2, 0xB2, '-', '4', 'a', 'c', ')', ')', '/', '(', '2', 'a', ')',
+        ' ', ' ', 0xE6, 0x97, 0xA5, 0xE6, 0x9C, 0xAC, 0xE8, 0xAA, 0x9E, ',',
+        0xE4, 0xB8, 0xAD, 0xE6, 0x96, 0x87, ',',
+        0xED, 0x95, 0x9C, 0xEA, 0xB8, 0x80, '\n'
+      };
+    ASSERT (check (input, SIZEOF (input), expected, SIZEOF (expected)) == 0);
+  }
+
+  /* String with characters outside the BMP.  */
+  {
+    static const uint16_t input[] =
+      { '-', '(', 0xD835, 0xDD1E, 0x00D7, 0xD835, 0xDD1F, ')', '=',
+        0xD835, 0xDD1F, 0x00D7, 0xD835, 0xDD1E
+      };
+    static const uint8_t expected[] =
+      { '-', '(', 0xF0, 0x9D, 0x94, 0x9E, 0xC3, 0x97, 0xF0, 0x9D, 0x94, 0x9F,
+        ')', '=', 0xF0, 0x9D, 0x94, 0x9F, 0xC3, 0x97, 0xF0, 0x9D, 0x94, 0x9E
+      };
+    ASSERT (check (input, SIZEOF (input), expected, SIZEOF (expected)) == 0);
+  }
+
+  /* Invalid input.  */
+  {
+    static const uint16_t input[] = { 'x', 0xDD1E, 0xD835, 'y' };
+#if 0 /* Currently invalid input is rejected, not accommodated.  */
+    static const uint8_t expected[] =
+      { 'x', 0xEF, 0xBF, 0xBD, 0xEF, 0xBF, 0xBD, 'y' };
+    ASSERT (check (input, SIZEOF (input), expected, SIZEOF (expected)) == 0);
+#else
+    size_t length;
+    uint8_t *result;
+    uint8_t preallocated[10];
+
+    /* Test return conventions with resultbuf == NULL.  */
+    result = u16_to_u8 (input, SIZEOF (input), NULL, &length);
+    ASSERT (result == NULL);
+    ASSERT (errno == EILSEQ);
+
+    /* Test return conventions with resultbuf too small.  */
+    length = 1;
+    result = u16_to_u8 (input, SIZEOF (input), preallocated, &length);
+    ASSERT (result == NULL);
+    ASSERT (errno == EILSEQ);
+
+    /* Test return conventions with resultbuf large enough.  */
+    length = SIZEOF (preallocated);
+    result = u16_to_u8 (input, SIZEOF (input), preallocated, &length);
+    ASSERT (result == NULL);
+    ASSERT (errno == EILSEQ);
+#endif
+  }
+
+  return 0;
+}