diff --git a/icons/mkicon.py b/icons/mkicon.py
index db535b93..d1bca2fa 100755
--- a/icons/mkicon.py
+++ b/icons/mkicon.py
@@ -901,9 +901,9 @@ def testrun(func, fname):
     for canvas in canvases:
         minx, miny, maxx, maxy = bbox(canvas)
         block.extend(render(canvas, minx-2, miny-2, minx-2+wid, maxy+2))
-    with open(fname, "w") as f:
-        f.write(("P7\nWIDTH %d\nHEIGHT %d\nDEPTH 3\nMAXVAL 255\n" +
-                 "TUPLTYPE RGB\nENDHDR\n") % (wid, ht))
+    with open(fname, "wb") as f:
+        f.write((("P7\nWIDTH %d\nHEIGHT %d\nDEPTH 3\nMAXVAL 255\n" +
+                  "TUPLTYPE RGB\nENDHDR\n") % (wid, ht)).encode('ASCII'))
         assert len(block) == ht
         for line in block:
             assert len(line) == wid
@@ -912,7 +912,7 @@ def testrun(func, fname):
                 r = int(round((r * a + 255 * (255-a)) / 255.0))
                 g = int(round((g * a + 128 * (255-a)) / 255.0))
                 b = int(round((b * a +   0 * (255-a)) / 255.0))
-                f.write("%c%c%c" % (r,g,b))
+                f.write(bytes(bytearray([r, g, b])))
 
 def drawicon(func, width, fname, orangebackground = 0):
     canvas = func(width / 32.0)
@@ -921,9 +921,10 @@ def drawicon(func, width, fname, orangebackground = 0):
     assert minx >= 0 and miny >= 0 and maxx <= width and maxy <= width
 
     block = render(canvas, 0, 0, width, width)
-    with open(fname, "w") as f:
-        f.write(("P7\nWIDTH %d\nHEIGHT %d\nDEPTH 4\nMAXVAL 255\n" +
-                 "TUPLTYPE RGB_ALPHA\nENDHDR\n") % (width, width))
+    with open(fname, "wb") as f:
+        f.write((("P7\nWIDTH %d\nHEIGHT %d\nDEPTH 4\nMAXVAL 255\n" +
+                  "TUPLTYPE RGB_ALPHA\nENDHDR\n") %
+                 (width, width)).encode('ASCII'))
         assert len(block) == width
         for line in block:
             assert len(line) == width
@@ -934,7 +935,7 @@ def drawicon(func, width, fname, orangebackground = 0):
                     g = int(round((g * a + 128 * (255-a)) / 255.0))
                     b = int(round((b * a +   0 * (255-a)) / 255.0))
                     a = 255
-                f.write("%c%c%c%c" % (r,g,b,a))
+                f.write(bytes(bytearray([r, g, b, a])))
 
 args = sys.argv[1:]