The photo-Fenton process (Fe²⁺ /Fe³⁺ , H₂O₂, UV light) is one of the most efficient and advanced oxidation processes for the mineralization of the organic pollutants of industrial effluents and wastewater. The overall rate of the photo-Fenton process is controlled by the rate of the photolytic step that converts Fe³⁺ back to Fe²⁺ . In this paper, the effect of sulfate or chloride ions on the net yield of Fe²⁺ during the photolysis of Fe³⁺ has been investigated in aqueous solution at pH 3.0 and 1.0 in the absence of hydrogen peroxide. A kinetic model based on the principal reactions that occur in the system fits the data for formation of Fe²⁺ satisfactorily. Both experimental data and model prediction show that the availability of Fe²⁺ produced by photolysis of Fe³⁺ is inhibited much more in the presence of sulfate ion than in the presence of chloride ion as a function of the irradiation time at pH 3.0.