The methylene blue photosensitized oxidation of cysteine sulfnic acid is investigated. Enhancement of the oxygen consumption rate in deuterium oxide suggests the involvement of singlet oxygen (O-1(2)) in oxidation, Addition of the O-1(2) quencher azide produced an unusual enhancement of the oxidation rate of all the sulfinates assayed. It is assumed that azide works as a one-electron carrier between O-1(2) and the sulfur compounds, Analyses of the products indicate that the photochemical oxidation of cysteine sulfinic acid proceeds through two simultaneous mechanisms, The Type II (singlet oxygen) mechanism is responsible for oxidation of the sulfinic group to the sulfonic group with production of cysteic acid, stable to the photooxidation system, whereas the Type I (electron transfer) mechanism is involved in the degradation of cysteine sulfinic acid to acetaldehyde. Other products detected were ammonia, sulfate, and hydrogen peroxide which account for the degradation of cysteine sulfinic acid and for the excess of oxygen consumption detected during the oxidative reaction.