Chloride ion (Cl-) is one of the most common anions in the aqueous environment. A mathematical model was developed to determine and quantify the impact of Cl- on the oxidization rate of organic compounds at the beginning stage of the UV/persulfate (PS) and UV/H2O2 processes. We examined two cases for the UV/PS process: (1) when the target organic compounds react only with sulfate radicals, the ratio of the destruction rate of the target organic compound when Cl- is present to the rate when Cl- is not present (designated as rRCl-/ rR) is no larger than 1.942%; and (2) when the target organic compounds can react with sulfate radicals, hydroxyl radicals and chlorine radicals, rRCl-/ rR, can be no larger than 60%. Hence, Cl- significantly reduces the organic destruction rate in the UV/PS process. In the UV/H2O2 process, we found that Cl- has a negligible effect on the organic-contaminant oxidation rate. Our simulation results agree with the experimental results very well. Accordingly, our mathematical model is a reliable method for determining whether Cl- will adversely impact organic compounds destruction by the UV/PS and UV/H2O2 processes.