The effect of Cl- on photocatalytic degradation(PCD) of pollutants is an important factor since it is ubiquitous in nature. In examine the effects of Cl- on the PCD of Methylene Blue (MB) and Orange Ⅱ (Oll) in aqueous TiO2 suspensions under UV light illumination. It was found that low concentration of Cl- ( < 0.01 mol/L) showed little influence on both dyes, however, high concentration of Cl- ( > 0.10 mol/L) had a very different influence on the decolorization of dyes: a significant inhibition for MB but a great promotion for Oll. In the presence of 0.50 mol/L Cl- , the rate decreased by 70% for MB while increased 7.5-fold for Oll. Furthermore, two bands in the ultraviolet region of Oll were rapidly broken down. The proposed mechanism was discussed in detail.