The kinetics and mechanism for methiocarb (MC) degradation by aqueous ClO2 were investigated under simulated water treatment conditions. Experimental results indicate that the reaction between MC and ClO2 was of second-order overall, and the rate constant rapidly increased from 0.56 to 4.5 M(-1) s(-1) as the solution pH increased from 6.0 to 9.1 at 23 degrees C. The activation energy was determined to be 75 kJ mol(-1) in the studied temperature range of 7-35 degrees C. Methiocarb sulfoxide (MCX) and methiocarb sulfone (MCN) were quantified to be the major byproducts from MC degradation. Unlike the sequential formation of sulfoxide and sulfone during the oxidation of many thioethers, the two byproducts were formed simultaneously during MC degradation by ClO2. The solution pH significantly affected the type and quantity of the degradation byproducts. For example, at pH 6.5 MCX and MCN accumulated as the reaction proceeded and finally accounted for 71% and 28% of MC degraded, respectively; while at pH 8.6 three more minor byproducts were identified. Though ClO2 can effectively oxidize MC in water, the significant increase in toxicity raises a potential risk to consumers.