This study elucidates the decolorization of C.I. Reactive Red 2 (RR2) in ultrasound (US), US/TiO2, ultraviolet (UV)/TiO2 and UV/US/TiO2 systems. The effects of TiO2 dosage, pH and temperature on RR2 decolorization were determined in a UV/US/TiO2 system. The pseudo-first-order decolorization rate constants of 0.5, 1 and 2 g/l TiO2 in the UV/US/TiO2 system were 0.45, 0.57 and 0.94 h(-1), respectively. In the UV/US/TiO2 system, the decolorization rate of RR2 declines as pH increases, and increases as temperature increasing. At pH 7, the decolorization rates followed the order of UV/US/TiO2 (0.94 h(-1)) > UV/TiO2 (0.85 h(-1)) > US/TiO2 (0.25 h(-1)) > US. The activation energy of the UV/US/TiO2 system at pHs of 4, 7 and 10 was 6.58. 11.62 and 21.32 kJ/mol, respectively. Experimental results suggest that the RR2 decolorization in the UV/US/TiO2 system was a diffusion-controlled reaction. (C) 2009 Elsevier B.V. All rights reserved.