The photocatalytic degradation of malachite green (MG) dye molecules in aqueous solution was investigated by using palladium (Pd) modified tungsten trioxide (WO3) under simulated solar light. The optimum values for Pd content vs. WO3 and catalyst concentration in solution for MG (5.0 mu mol L-1) degradation were 0.5 wt.% and 150 mg L-1. respectively. The MG concentration change followed the pseudo first order kinetics of the Langmuir-Hinshelwood model. Since MG was also degraded under visible light (lambda>470 nm), which was not absorbed by WO3, the mechanism involved both the photocatalytic degradation and self-sensitized degradation of MG. Pd modified WO3 would be useful as an efficient tool for the decolorization of wastewater under solar light. (C) 2010 Elsevier B.V. All rights reserved.