A model system consisting of casein (nine parts) and methyl linoleate (one part) has been used to study changes in methionyl residues of proteins as a consequence of lipid oxidation. A modified McCarthy-Sullivan procedure was developed which could distinguish methionine from its oxidation products, since acid hydrolysis partially converts methionine sulfoxide back to methionine. The concentration of unreacted methionine was followed in the model system at storage relative humidities of 0, 33, and 75%; in each case the loss of methionine was proportional to the amount of proteinbound nonenzymatic browning pigment. Methionyl residues may act as peroxide decomposers with concomitant carbonyl compound formation which in turn would lead to nonenzymatic browning. © 1969, American Chemical Society. All rights reserved.