Nano cerium oxides are efficient photocatalysts for pollutants degradation with highly dispersed Ce(III) ions as the suggested active species to promote the reaction, while Ce(IV) species do not behave as a catalyst. In this paper, to understand the mechanism of Ce-based photocatalysts, we studied the comparison of simple cerium ions, Ce(III) and Ce(IV) in aqueous solution for organic pollutants degradation under UV irradiation. Orange II(AOII), methyl orange, and p-nitrophenol were selected as the target pollutants. The formation and contribution of reactive oxygen species, the kinetics of Ce(IV) photoreduction and Ce(III) photooxidation, and the influence of solution p H were investigated in detail. It was found that at low p H Ce(IV) ions showed a higher activity for hydroxyl radicals production and AOII degradation than Ce(III) ions, which could be attributed to its fast reduction rate to Ce(III). However, its activity dramatically decreased when solution p H increased, and was also strongly influenced by the type of pollutants; while Ce(III) exhibited high degradation efficiency of all the tested pollutants over a wide pH range. Nano cerium olyols are efficient photocatalysts for pollutants degradation with highly dispersed Ce (III) ions as the suggested active species to promote the reaction, while Ce (IV) species do not behave as a catalyst. In this paper, to understand the mechanism of Ce -based photocatalysts, we studied the comparison of simple cerium ions, Ce (III) and Ce (IV) in aqueous solution for organic pollutants degradation under UV irradiation. Orange II (AOII), methyl orange, and p-nitrophenol were selected as the The formation and contribution of reactive oxygen species, the kinetics of Ce (IV) photoreduction and Ce (III) photooxidation, and the influence of solution p H were investigated in detail. It was found that at low p H Ce (IV ) ions showed a higher activity for hydroxyl radical production and AOII degradation than Ce (III) ions, which could be attributed to its fast reduction rate to Ce (III). However, its activity dramatically decreased when solution p H increased, and was als strongly influenced by the type of pollutants; while Ce (III) exhibited high degradation efficiency of all the testing pollutants over a wide pH range.