The kinetics of the reduction of colloidal MnO2 by ascorbic acid has been studied spectrophotometrically. Under pseudo-first-order conditions ([ascorbic acid] >> [(MnO2)(n)] the reaction was very fast. To obtain the rate constants as a function of [ascorbic acid], [(MnO2)(n)], and temperature, second-order conditions were maintained in each kinetic run. The rate of the reduction increases and decreases with the increase in [ascorbic acid] and [(MnO2)(n)]), respectively. The reaction was catalyzed by perchloric acid. The addition of sodium pyrophosphate and sodium fluoride has inhibitory effect on the reaction rate. Externally added Mn(II) (a product of the reaction) has composite effect (inhibition and catalytic). The effect of gum arabic has also been studied. Activation parameters have been evaluated from the Arrhenius and Eyring equations. The kinetic evidence does not indicate the formation of an intermediate complex between colloidal MnO2 and ascorbic acid. A mechanism consistent with the observed kinetics has been proposed and discussed. (c) 2005 Elsevier Inc. All rights reserved.