Measurements of oxygen absorption at 50° C showed methylene blue to be the most effective of several sensitizers tested. At a concentration of 0.01 mM, it gave a rate of oxidation which was about 12 times that in a control run in the light. About 1600 moles of oxygen were absorbed per mole of methylene blue. To obtain the same rate in the dark, about 100 times as much 2,2′-azobis(2-methyl-propionitrile) (AZBN) would have been required. At least in the initial stages of oxidation, this catalysis by methylene blue was not affected by addition of an oxidation inhibitor. Thus, the photosensitized oxidation is mostly a nonradical reaction.