Department of Chemistry, University of Karachi, Karachi-75270, Pakistan E-mail: abida2020@yahoo.com Manuscript received 15 March 2004, revised 28 July 2004, accepted 23 August 2004 The kinetics of the redox reaction between diphenylbenzidine (DPB) and thiosulfate was investigated in the range pH 2.0 to 4.5. Order of reaction with respect to thiosulfate, diphcnylbenzidine and H+ was found to be one. The following rate law is recommended : Rate= k1 [DPB] [S2O2-3] + k2 K {DPB [H+]} [S2O2-3] where k1, k2 are the rate constants for the reaction [DPB] + S2O2-3 → [S4O2-6] + Product having the values of k1 = 5.557 x 103 dm3 mol-1 s-1 and k2 = 5.827 x 108 dm3 mol-1 s-1 K is the protonation constant of DPB having value of 476 dm3 mol-1.