The kinetics Of UV/TiO2 photocatalytic reduction of Se(VI) in the presence of formic acid was investigated. Competitive adsorption models for the Se(VI) and HCOO- ions onto the same active sites on TiO2 surface were derived, based on the adsorption of one Se(VI) ion onto two adsorptive sites. The use of the above adsorption models allows for the modeling of the Se(VI) photoreduction rates on the basis of the Langmuir-Hinshelwood (LH) reaction mechanism. The new model was able to represent the experimental data reasonably well, and it supported the experimental observation that the optimum Se(VI) photoreduction rate occurred at the molar adsorption ratio of formate-to-Se(VI) ions of approximately 3:1. A composite rate expression incorporating the effect of catalyst loading was also presented.