Gas phase IR spectra in the 800–2000 cm−1 range of protonated methionine and its sulfoxide were recorded and compared to those computed for the lowest energy structures. The signature of the SO bond was clearly identified at around 1000 cm−1. Oxidation of methionine–lysine dipeptide by OH radicals in the presence of catalase revealed the formation of methionine sulfoxide upon one-electron oxidation.