Thanks to all responders for the information. Conclusion is that it is possible to rely on the size and alignment of CRITICAL_SECTION not changing. I have to implement this edit to the library for my own purposes, as windows.h is conflicting with too much code (including other libraries which I do not want to modify). Ray's point about MIN and MAX macros is well taken. These macros caused me problems too. I did not mention them because it is possible to avoid by defining NOMINMAX in the project file. The Boost documentation does say that the use of Windows critical sections is an interim measure pending a portable Boost implementation of equivalent features. This means that it may not be worth making any changes to the library until the job can be done properly. I suspect this won't be too long as Boost has recently increased it's portable support for thread synchronisation. regards, Paul