diff --git a/nc_cxx.cmake b/nc_cxx.cmake
index b58b30c..b2da366 100644
--- a/nc_cxx.cmake
+++ b/nc_cxx.cmake
@@ -5,6 +5,14 @@ endif ()
 
 set (__nc_cxx TRUE)
 
+###############################################################################
+# clear the default flags because they include optimisation flags we
+# explicitly do not want (ie, O3).
+set (CMAKE_CXX_FLAGS_RELEASE "")
+set (CMAKE_C_FLAGS_RELEASE "")
+set (CMAKE_CXX_FLAGS_DEBUG "")
+set (CMAKE_C_FLAGS_DEBUG "")
+
 
 ###############################################################################
 include (compile_flag)