A kinetic model containing three parameters was developed based upon mechanistic considerations to include the effect of dissolved oxygen over a range of conditions in aqueous media. It shows that as organic concentrations increase to concentrations on the order of the solubility of oxygen concentration in water (similar to1.26 mM at oxygen partial pressures of I atm), the solubility of oxygen in water limits the extent of photo-oxidation of the organic substrate. The model reduces to the commonly used Langmuir-Hinshelwood kinetic rate expression for the photo-oxidation of organic substrates in aqueous media under constant oxygen concentrations, which is an appropriate assumption when oxygen is initially in large excess of the organic substrate or when the reaction mixture is aerated during the reaction. The model was fit to the experimental data generated in this study for the photo-oxidation of phenol to determine the best values for the model parameters. For comparison, the model was also fit to selected published data for the photo-oxidation of phenol.