Measurements of kinetic isotope-effects (k*/k) for the oxidation of ten aldoses-1-t with sodium chlorite in acid solution gave values ranging from 0.56 to 0.75, with an average of 0.66. The values show that the C1—H* bond is not ruptured in the rate-determining step. For the reaction, a mechanism is proposed which accounts for this fact and for the dependence of the rate of reaction on the concentration of chlorous acid. The mechanism involves formation of a chlorous acid—aldehyde intermediate; this decomposes, giving the aldonic acid and hypochlorous acid. The latter then reacts with more chlorous acid, affording the chlorine dioxide and hydrogen chloride found experimentally.