Chlorine is widely used as a disinfectant in water treatment and in the food industry. Five organophosphorus pesticides (EPN, diazinon, fenitrothion, malathion and parathion) were, therefore, subjected to aqueous chlorination with hypochlorite to evaluate inhibitory effect on cholinesterase (ChE) activity and mutagenic effect of these reaction products. Treatment with hypochlorite of thiophosphorus (P=S) pesticide solutions was shown to produce compounds with a great inhibiting effect on ChE activity, but, further reactions resulted in decrease in the ChE inhibiting activity. Aqueous chlorination with hypochlorite of these P=S compound solutions was also shown to produce compounds with mutagencity onS. typhimuriumTA98 without S9 mix. The production of mutagens is dependent on the parent pesticides, the solution pH values, and the reaction time.