The optimum conditions for UV-photocatalytic degradation of paraquat over titanium dioxide supported on rice husk silica (TiO(2)/RH-SiO(2)) catalyst, which is effected by four independent variables, namely initial paraquat concentration, pH of solution, titanium dioxide content (% TiO(2)) and catalyst loading, have been evaluated. The TiO(2)/RH-SiO(2) catalyst has been synthesized by colloidal impregnation method and calcined at 550 degrees C for 6 h. The Box-Behnken design, based on response surface methodology has been applied to design the experiment and analyze the data. Characterization of the catalysts is investigated by XRD, SEM, BET surface area and UV-DRS for explanation of reaction behaviour. The XRD patterns show a pure anatase crystalline phase at all TiO(2) percentages, and the BET surface area of the catalysts is vastly decreased as the percentage of TiO(2) is increased. The highest paraquat removal efficiency of 90.04% is obtained at 10 ppm initial paraquat concentration, 5.91 pH, 30 wt % TiO(2) and 2.0 g/L catalyst loading.