The study examined the effect of operating conditions of the Fenton process on degradation, biodegradability improvement and mineralization of chlorpyrifos, cypermethrin and chlorothalonil pesticides in aqueous solution. Also, the degradation of the pesticides under optimum operating conditions was evaluated. The optimum operating conditions for Fenton treatment of an aqueous solution containing 100, 50 and 250 mg L-1 of chlorpyrifos, cypermethrin and chlorothalonil, respectively were observed to be H202/C0D molar ratio 3, H202/Fe2+ molar ratio 10 and pH 3. Pesticide degradation increased with temperature. Under the optimum operating conditions, complete degradation of the pesticides occurred in 1 min. Biodegradability (BOD5/COD ratio) increased from zero to 0.35 and COD and TOC removal were 69.03 and 55.61, respectively in 60 min. Due to mineralization of organic carbon, decrease in ammonia-nitrogen from 22-6.1 mg L-1 and increase in nitrate from 0.7-15.2 mg L-1 in 60 min were observed. The study indicated that Fenton process can be used for pretreatment of combined chlorpyrifos, cypermethrin and chlorothalonil aqueous solution or wastewater for biological treatment. © 2012 Asian Network for Scientific Information.