Abstract The methylene-blue-sensitized photo-oxidation of 2-imidazolidinethione (IT) in water gives 2-imidazolin-2-yl-sulphinate as the primary and only product. The quantum yield φ of photo-oxidation increases and then decreases to a zero value during the course of the reaction. The dye undergoes photobleaching in the absence of oxygen. A mechanism involving interaction of the triplet dye with O 2( 3Σ g −) and/or IT and leading to products through the participation of IT + • and O 2 − is suggested. The effect of singlet oxygen quenchers is discussed.