COMPex Pro 50 excimer KrF laser with wavelength = 248 nm is used to monitor conformational changes during protein folding and function. With this method, OH radicals are generated from H2O2 by a pulsed KrF excimer laser (248 nm) and used to modify irreversibly solvent-exposed sites in the protein. The short lifetime of the OH radical means that labelling occurs more rapidly than most conformational changes, allowing a snapshot of the protein conformation at any stage of a reaction to be analysed (with appropriate scavengers the reaction time is ~1 microsec). Most amino acid side-chains can be modified if exposed and oxidative labelling results in a mass increase of 16 Da per site modified, which is readily detectable using mass spectrometry.