Sodium azide is widely used as bacteriostatic agent during downstream processing of proteins. Amino acid composition analysis of protein samples, subjected to hydrolysis with hydrochloric acid in a buffer containing sodium azide, revealed the presence of cysteic acid, methionine sulfoxide, and methionine sulfone in addition to the expected reaction products. Hydrolysis with methanesulfonic acid in the presence of sodium azide resulted in detection of only methionine sulfoxide in addition to the expected products. When the proteins were hydrolyzed in a buffer containing no sodium azide or after its removal by dialysis, no oxidation products were detected (except for minor amounts of methionine sulfoxide). The generation of the particular oxidation products was affected by the concentration of sodium azide in the protein solution. Therefore, presence of sodium azide in protein samples intended for amino acid composition analysis may lead to wrong conclusions concerning oxidation of cysteine and methionine residues.