Hi Martin and Angelo, You have probably now seen that I have just released relax 2.2.4 (http://article.gmane.org/gmane.science.nmr.relax.user/1413) - I would recommend you upgrade as this fixes the Rex values of zero from the value.write user function. You'll see that the documentation for the value.write user function now describes these small values (http://www.nmr-relax.com/manual/value_write.html). This will still be confusing for the user as you would still need to read the documentation, so I might look at adding a small comment to the top of each file describing the parameter. What do you think of the following idea? The parameter descriptions could be added to the top of the file as a comment, for example see the current model-free descriptions at: http://www.nmr-relax.com/api/specific_fns.model_free-pysrc.html Specifically the 'desc=' arguments to the self.PARAMS.add() calls which are normally only used in the XML relax state files. These descriptions could then be used to add the following text to the top of the file: # Parameter description: Chemical exchange relaxation (sigma_ex = Rex / omega**2). Do you think this would be sufficiently clear? If not, could you see how the description can be improved? Or would you have a better idea as to how to convey this information? Cheers, Edward