The kinetics of 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation by sodium periodate in an aqueous solution was studied. For the auto-acceleration regime, the experimental data correspond to the kinetic equation w t = k[P][IO] [TMB]0, where w t is the accumulation rate of the meriquinoid product (P) of TMB oxidation and [P]t and [IO]t are the concentrations of product P and periodate, respectively, at time t. A radical chain mechanism was proposed; the mechanism explains the experimental kinetic equation and complies with the observed inhibiting effect of metal ions (Zn, Cd) in this reaction. [ABSTRACT FROM AUTHOR]