diff --git a/library/alignment.h b/library/alignment.h
index 4aab8e0275..634d610469 100644
--- a/library/alignment.h
+++ b/library/alignment.h
@@ -224,10 +224,25 @@ static inline uint64_t mbedtls_bswap64(uint64_t x)
 #endif /* !defined(MBEDTLS_BSWAP64) */
 
 #if !defined(__BYTE_ORDER__)
+
+#if defined(__LITTLE_ENDIAN__)
+/* IAR defines __xxx_ENDIAN__, but not __BYTE_ORDER__ */
+#define MBEDTLS_IS_BIG_ENDIAN 0
+#elif defined(__BIG_ENDIAN__)
+#define MBEDTLS_IS_BIG_ENDIAN 1
+#else
 static const uint16_t mbedtls_byte_order_detector = { 0x100 };
 #define MBEDTLS_IS_BIG_ENDIAN (*((unsigned char *) (&mbedtls_byte_order_detector)) == 0x01)
+#endif
+
 #else
-#define MBEDTLS_IS_BIG_ENDIAN ((__BYTE_ORDER__) == (__ORDER_BIG_ENDIAN__))
+
+#if (__BYTE_ORDER__) == (__ORDER_BIG_ENDIAN__)
+#define MBEDTLS_IS_BIG_ENDIAN 1
+#else
+#define MBEDTLS_IS_BIG_ENDIAN 0
+#endif
+
 #endif /* !defined(__BYTE_ORDER__) */
 
 /**