The effect of dissolved oxygen on the photocatalytic reduction of Cr(VI) with illuminated TiO₂ was studied in this work with variation of the solution pH, contact time, and initial Cr(VI) concentration. Oxygen or nitrogen gas was used as a purging gas. Overall, the removal efficiency of Cr(VI) decreased as the solution pH increased. The removal of Cr(VI) by UV/TiO₂ increased by decreasing the solution pH because of the increased potential difference between the conduction band of TiO₂ and Cr(VI)/Cr(III) as well as the anionic-type adsorption of Cr(VI) onto the TiO₂ surface. The removal efficiency of Cr(VI) increased in purging of nitrogen gas compared to that of oxygen gas because of less competition between dissolved oxygen and Cr(VI) in the reaction with the electron in the conduction band of TiO₂. The reduction pattern was better described by the first-order kinetic model.