Effects of acidity and inorganic ions that are common in industrial effluent on the photocatalytic degradation of azo dyes, Procion Red MX-5B (MX-5B) and Cationic Blue X-GRL (CBX), have been investigated in UV illuminated TiO2 dispersions. There are significant differences between adsorption and photodegradation of MX-5B and those of CBX to the change of solution pH. The results indicated that CBX photodegradation was favored at the surface of TiO2, while that of MX-5B occurred in the aqueous phase. At pH 2.4, SO42-, H2PO4-, ClO4- and F- in general increased the decolorization rates of MX-5B and CBX by enhancing the adsorption of the selected dyes on the surface of TiO2. At pH 10.8, most of the selected anions inhibited the photocatalytic oxidation (PCO) to decolorize and degrade CBX and MX-5B. These results demonstrated that inorganic anions affect the photodegradation of dyes by their adsorption onto the surface of TiO2 and trapping positive hole (h(+)) and (OH)-O-.. Inorganic cationic ions, such as Cu2+ and Ni2+ had strong inhibition on the PCO decolorization of MX-5B at pH 10.8. On the contrary, no significant effect was observed at the same concentration of metal cations at pH 2.4. (C) 2003 Elsevier Science B.V. All rights reserved.