The collisionally induced loss of NO2 of protonated nitroimidazoles and nitropyrazoles was investigated for the possible generation of hydrogen shift isomers of imidazole and pyrazole radical cations. Ab initio calculations at B3LYP/6-31+G(d,p) level have been used to rationalize the experimental findings. (C) 2002 Published by Elsevier Science B.V.