diff --git a/CGAL_ImageIO/include/CGAL/ImageIO_impl.h b/CGAL_ImageIO/include/CGAL/ImageIO_impl.h
index 8dbef016b1c5..fefbd0d4cc8d 100644
--- a/CGAL_ImageIO/include/CGAL/ImageIO_impl.h
+++ b/CGAL_ImageIO/include/CGAL/ImageIO_impl.h
@@ -1242,7 +1242,7 @@ static void _swapImageData( _image *im )
    Swap bytes depending on the endianness and the current architecture  */
 CGAL_INLINE_FUNCTION
 int _readImageData(_image *im) {
-  unsigned long size, nread;
+  std::size_t size, nread;
 
   if(im->openMode != OM_CLOSE) {
     size = im->xdim * im->ydim * im->zdim * im->vdim * im->wdim;
@@ -1279,7 +1279,7 @@ int _readImageData(_image *im) {
    Swap bytes depending on the endianness and the current architecture. */
 CGAL_INLINE_FUNCTION
 int _readNonInterlacedImageData(_image *im) {
-  unsigned long size, nread;
+  std::size_t size, nread;
   unsigned char **vp, *buf;
   unsigned int i, j, k, v, w;
 
@@ -1356,7 +1356,7 @@ int _readNonInterlacedImageData(_image *im) {
    been read by _readImageHeader. The image buffer is interlaced. */
 CGAL_INLINE_FUNCTION
 int _readNonInterlacedFileData(_image *im) {
-  unsigned long size, nread;
+  std::size_t size, nread;
   unsigned char *ptr1, *vp, *buf;
   unsigned int i, j, k, v, w;