Hie photochemical reduction of thionine by N-phenyl glycine, N-allylurea, N -phonylurea and thiourea in absolute me¬ thanol ms investigated. An apparatus for the irradiation of air free solutions, with known intensities of monochromatic light was constructed. Double walled glass reaction cell was deviced in which nitrogen inlet/outlet, water inlet/outlet ar¬ rangements were made. Acidities of the reaction solutions were controlled using buffer solutions. Acidities determined spec- trophotometrically using basic indicator were expressed in terms of a Hamnett acidity function. Variations of quantum ef¬ ficiencies in absolute methanol have been determined at various concentrations of thionine, reductants, and acidities. The oxy¬ gen free reaction solutions were irradiated and the react ions were followed by measuring the intensity of light transmitted. The quantum efficiency, in yellow light, for the dis¬ appearance of thionine, was found to be a function of reductant concentration, acidity and temperature. The quantum efficiency was found to be independent of the concentration of thionine. The results have been interpreted in terms of a reaction mechanism Ratios of the certain rate constants for the proposed mec banian have been evaluated. It was found that the quantum efficiency is controlled by two equilibriums between: i) triplet state of thionine with proton, and protonated triplet state of thionine, and ii) protonated triplet state of thionine with reductant, and associated complex of thionine with reductant.