Studies have been made on the reactivity of sulfur entities in oxidized asphalts and oxidized petroleum distillation residues during air oxidation. Asphalts are referred to as residues. The oxidation of these materials includes the rapid formation of an unsymmetrical ir band at ca 1030/cm. Sulfoxides are not normally found in unoxidized petroleum residues, but develop rapidly during air aging. Their presence in these residues has been inferred from their specific bands in ir spectra. Confirmation of sulfoxides was obtained from deoxygenation with chlorocarbenes and decomposition with bromine and peracetic acid. Hydroperoxides in amounts that might interfere with sulfoxide determination were not found in oxidated residues. The concentration of sulfoxides formed during air oxidation was found to rapidly reach a maximum value and to remain constant on further oxidation of the residue. Model studies indicate that hydroperoxides play a role in sulfoxide formation. The data show that both alkyl and aryl sulfides were present in aliphatic residues, that predominantly alkyl types were oxidized during oxidative aging in air and that concentrations of the alkyl and aryl types vary with crude source. Peracetic acid oxidation of the residues indicated that most of the sulfur present was in the form of sulfides. 2 figures, 6 tables.