A kinetic study of the reaction between colloidal manganese dioxide and oxalic acid in aqueous acetate media (pH 4.3-5.1) is reported. The reaction is autocatalytic and, in order to determine the rate constants k(1) and k(2) corresponding, respectively, to the noncatalytic and autocatalytic reaction pathways, the Mn(III) formed as an intermediate was stabilized by addition of sodium pyrophosphate. Joint iodimetric and spectrophotometric studies indicated that the reduction of colloidal manganese dioxide follows the sequence Mn(IV) --> R Mn(II) --> Mn(IV) Mn(III) --> R (II), where R stands for the reductant. Both reaction pathways exhibit acid catalysis, and the activation energies associated to k(1) and k(2) are 74.7 +/- 0.9 and 44.6 +/- 0.6 kJ mol(-1), respectively. The reaction is accelerated by addition of manganese(II) sulfate, and k(2) increases with rising oxalate concentration, suggesting that MnC2O4 is the active autocatalyst. The possibility that Mn(III) might collaborate in the autocatalysis through a free-radical chain mechanism is also pointed out.