In this paper, the effects of various inorganic anions (NO3−, Cl−, SO4 2−, HCO3−, H2PO4−) on the photodegradation of an azo dye, acid orange 7 (AO7) were investigated under UV or visible light irradiation in the presence of TiO2 particles. The effects of inorganic anions on the photodegradation of AO7 under visible light irradiation were significantly different from UV irradiation pathway. In TiO2/UV system, inorganic anions inhibited the photodegradation of the dye by their trapping hydroxyl radicals. In TiO2/Vis system, the observed inhibition effects of inorganic anions can be interpreted by competitive adsorption. In addition, the results indicated that the photodegradation of AO7 took place mainly in the bulk solution under UV light irradiation, while under visible light irradiation, the reaction occurred on the catalyst surface.