The kinetics of the reduction of diphenylbenzidine by ascorbic acid were investigated in the pH range 3.5 to 5.0. Diphenylbenzidine was prepared by the oxidation of diphenylamine with cerium(IV) sulfate tetra hydrate. The order of the reaction with respect to each of the reactants, i.e ascorbic acid (ASC), diphenylbenzidine (DPB) and H+ was found to be first. The following rate law is recommended: Rate = {k(1) [C12H10N2C12H10} + k(2)K [C12H10N2C12H10][H+]}[C6H8O6] where k(1) and k(2) have the values of 2.0 x 10(4) dm(3) mol(-1) s(-1) and 6.58 x 10(3) dm(3).mol(-1).s(-1), respectively. K is the protonation constant of DPB, having a value of 476 dm(3) mol(-1). A possible mechanism for the reaction is also proposed.