diff --git a/src/tests/testio.c b/src/tests/testio.c
new file mode 100644
index 00000000..0226c0e3
--- /dev/null
+++ b/src/tests/testio.c
@@ -0,0 +1,148 @@
+/* vim:expandtab:ts=2 sw=2:
+*/
+/*  Grafx2 - The Ultimate 256-color bitmap paint program
+
+    Copyright 2018-2020 Thomas Bernard
+    Copyright 2011 Pawel Góralski
+    Copyright 2009 Petter Lindquist
+    Copyright 2008 Yves Rizoud
+    Copyright 2008 Franck Charlet
+    Copyright 2007-2011 Adrien Destugues
+    Copyright 1996-2001 Sunset Design (Guillaume Dorme & Karl Maritaud)
+
+    Grafx2 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; version 2
+    of the License.
+
+    Grafx2 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 Grafx2; if not, see <http://www.gnu.org/licenses/>
+*/
+///@file testio.c
+/// Unit tests.
+///
+
+#include "tests.h"
+#include "../struct.h"
+#include "../io.h"
+#include "../gfx2log.h"
+
+int Test_Read_Write_byte(void)
+{
+  char path[256];
+  FILE * f;
+  byte b = 0;
+
+  snprintf(path, sizeof(path), "%s%stmp.bin", tmpdir, PATH_SEPARATOR);
+  f = fopen(path, "w+b");
+  if (f == NULL)
+  {
+    GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+    return 0;
+  }
+  if (!Write_byte(f, 42))
+  {
+    GFX2_Log(GFX2_ERROR, "error writing\n");
+    fclose(f);
+    return 0;
+  }
+  rewind(f);
+  if (!Read_byte(f, &b))
+  {
+    GFX2_Log(GFX2_ERROR, "error reading\n");
+    fclose(f);
+    return 0;
+  }
+  if (b != 42)
+  {
+    GFX2_Log(GFX2_ERROR, "Byte value mismatch\n");
+    fclose(f);
+    return 0;
+  }
+  fclose(f);
+  Remove_path(path);
+  return 1;
+}
+
+int Test_Read_Write_word(void)
+{
+  char path[256];
+  FILE * f;
+  word w1 = 0, w2 = 0;
+  byte b = 0;
+
+  snprintf(path, sizeof(path), "%s%stmp.bin", tmpdir, PATH_SEPARATOR);
+  f = fopen(path, "w+b");
+  if (f == NULL)
+  {
+    GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+    return 0;
+  }
+  // write bytes 12 34 56 78 90
+  if (!Write_word_le(f, 0x3412) || !Write_word_be(f, 0x5678) || !Write_byte(f, 0x90))
+  {
+    GFX2_Log(GFX2_ERROR, "error writing\n");
+    fclose(f);
+    return 0;
+  }
+  rewind(f);
+  if (!Read_byte(f, &b) || !Read_word_be(f, &w1) || !Read_word_le(f, &w2))
+  {
+    GFX2_Log(GFX2_ERROR, "error reading\n");
+    fclose(f);
+    return 0;
+  }
+  if ((b != 0x12) || (w1 != 0x3456) || (w2 != 0x9078))
+  {
+    GFX2_Log(GFX2_ERROR, "word values mismatch\n");
+    fclose(f);
+    return 0;
+  }
+  fclose(f);
+  Remove_path(path);
+  return 1;
+}
+int Test_Read_Write_dword(void)
+{
+  char path[256];
+  FILE * f;
+  dword dw1 = 0, dw2 = 0;
+  byte b = 0;
+
+  snprintf(path, sizeof(path), "%s%stmp.bin", tmpdir, PATH_SEPARATOR);
+  f = fopen(path, "w+b");
+  if (f == NULL)
+  {
+    GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+    return 0;
+  }
+  // write bytes 01 02 03 04 05 06 07 08 09
+  if (!Write_dword_le(f, 0x04030201) || !Write_dword_be(f, 0x05060708) || !Write_byte(f, 9))
+  {
+    GFX2_Log(GFX2_ERROR, "error writing\n");
+    fclose(f);
+    return 0;
+  }
+  rewind(f);
+  if (!Read_byte(f, &b) || !Read_dword_be(f, &dw1) || !Read_dword_le(f, &dw2))
+  {
+    GFX2_Log(GFX2_ERROR, "error reading\n");
+    fclose(f);
+    return 0;
+  }
+  if ((b != 0x1) || (dw1 != 0x02030405) || (dw2 != 0x09080706))
+  {
+    GFX2_Log(GFX2_ERROR, "word values mismatch\n");
+    fclose(f);
+    return 0;
+  }
+  fclose(f);
+  Remove_path(path);
+  return 1;
+}
+
diff --git a/src/tests/testlist.h b/src/tests/testlist.h
index 48ba2361..ca4b8482 100644
--- a/src/tests/testlist.h
+++ b/src/tests/testlist.h
@@ -1,6 +1,10 @@
 /* list of tests
  * TEST(function_to_test) */
 
+TEST(Read_Write_byte)
+TEST(Read_Write_word)
+TEST(Read_Write_dword)
+
 TEST(MOTO_MAP_pack)
 TEST(CPC_compare_colors)
 TEST(Packbits)