diff --git a/README b/README
index 9953040..69bc537 100644
--- a/README
+++ b/README
@@ -153,6 +153,13 @@ libburn.so.4    (since 0.3.4, March    2007),
 libisofs.so.6   (since 0.6.2, February 2008),
 libisoburn.so.1 (since 0.1.0, February 2008).
 
+Applications must use 64 bit off_t. E.g. by defining
+#define _LARGEFILE_SOURCE
+#define _FILE_OFFSET_BITS 64
+or take special precautions to interface with the libraries by 64 bit integers
+where the .h files prescribe off_t. Not to use 64 bit file i/o will keep the
+application from producing and processing ISO images of more than 2 GB size.
+
 
 ------------------------------------------------------------------------------
 Project history as far as known to me:
diff --git a/libburn/libburn.h b/libburn/libburn.h
index f2f8c50..56fba35 100644
--- a/libburn/libburn.h
+++ b/libburn/libburn.h
@@ -3,11 +3,15 @@
 #ifndef LIBBURN_H
 #define LIBBURN_H
 
-/* Needed for  off_t  which is the (POSIX-ly) appropriate type for
-   expressing a file or stream size.
+/* 
+
+Applications must use 64 bit off_t. E.g. by defining
+  #define _LARGEFILE_SOURCE
+  #define _FILE_OFFSET_BITS 64
+or take special precautions to interface with the library by 64 bit integers
+where this .h files prescribe off_t. Not to use 64 bit file i/o will keep the
+application from producing and processing ISO images of more than 2 GB size.
 
-   XXX we should enforce 64-bitness for off_t
-   ts A61101 : this is usually done by the build system (if it is not broken)
 */
 #include <sys/types.h>