In this study, the photocatalytic degradation of Direct Red 23 (Scarlet F-4BS) was investigated in UV/TiO2 system. The effect of catalyst loading and pH on the reaction rate was ascertained and optimum conditions for maximum degradation were determined. The results obtained showed that acidic pH is proper for the photocatalytic removal of Direct Red 23. In addition, the effects of several cations (Cu2+, Al3+,, Cr3+,, and Sn4+,) and anions (BiO3-, SO42-, and CN-) and C2H5OH were examined in this photocatalytic process. On the order hand, three types of catalysts (Fe2O3, SnO2, and ZnO) were compared with TiO2. After 90 min reaction, the relative decomposition order established was UV/TiO2 > UV/SnO2 > UV/Fe2O3 > UV/ZnO. (C) 2007 Elsevier B.V. All rights reserved.