Singlet oxygen (O-1(2)) mediated self-sensitized photo-oxidation in aqueous solution is a relatively uncommon photodegradation pathway that has been reported for some organic pollutants. Such reactions have been modeled by the empirical first-order kinetics, but significant deviation occurs at high substrate concentrations and in the presence of O-1(2) scavengers and in D2O matrices. A mechanistically based kinetic model was developed for O-1(2) mediated selfsensitized photo-oxidation by accounting for the sensitized generation of O-1(2) and the heterogeneous reaction between the substrate/sensitizer and O-1(2) in the photochemical system. The initially formed O-1(2) is treated in the model to be spatially correlated with its sensitizer, which could simultaneously undergo reaction and dissociation, instead of being evenly distributed in the solution instantaneously once formed. The oxidation of the substrate/sensitizer by O-1(2) occurs in both the geminate pairs and in solution, while the O-1(2) in solution is also quenched by the solvent and scavengers present. The model could well describe the solar photodegradation of p-arsanilic acid (p-ASA) under various initial concentrations, and predict the effect of O-1(2) scavenger NaN3, and D2O matrices on degradation rate. The performance of the model was also validated by the kinetics of O-1(2) mediated self-sensitized photo-oxidation of p-aminobenzoic acid (PABA), l-benzyl-3,4-dihydroisoquinoline, and 3,4-dihydropapaverine reported in the literature. Overall, this kinetic model could help better understand the fundamental processes involved in O-1(2) mediated self-sensitized photo-oxidation and predict the photochemical fate of organic pollutants that undergo such photochemical transformation in sunlit surface water.