Ferrioxalate is a visible light-responsive photocatalyst. The solar ferrioxalate/ H2O2 process has high degradation efficiency because ferrioxalate is able to absorb light strongly at longer wavelength and generates hydroxyl radical with high quantum yield. Degradation of pesticide chlorothalonil in aqueous solution by ferrioxalate/ H2O2 under solar irradiation was examined. The optimum operating conditions for treatment of a 300 mg/L chlorothalonil aqueous solution were obtained by using the central composite design of the response surface methodology. Under the optimum operating conditions (H2O2 /COD molar ratio 2.75, H2O2 /Fe3+ molar ratio 75, H2O 2 /Cmolar ratio 37.5, reaction time 90 min, and pH 3), COD, NHN, and TOC removal of 75.71, 47.11, and 54.33, respectively, was achieved and the biodegradability (BODCOD ratio) improved from zero to 0.42. Model prediction and actual removal were in close agreement (<4 error). The solar ferrioxalate/Hprocess is effective in pretreatment of the chlorothalonil aqueous solution for biological treatment. © 2013 Malay Chaudhuri et al.