There has been steady interest in the aqueous reaction of ClO2 center dot with sulfur(IV) since the 1950s, and a wide variety of rate laws and mechanisms have been proposed. In neutral-to-alkaline media, the reaction is challenging to study because of its great rate. Here it is shown that benzaldehyde can be used as an additive to slow the reaction and make its rates more amenable to study. The rates can be quantitatively modeled by a mechanism that includes reversible binding of sulfur(IV) by benzaldehyde and a rate-limiting mixed second-order reaction of ClO2 center dot with SO32-. The latter reaction occurs through parallel electron transfer from SO32- to ClO2 center dot and oxygen-atom transfer from ClO2 center dot to SO32-.