The reduction of 2,6-dichlorophenolindophenol (DCPI) by sulphides and sulphites has been studied kinetically by the stopped-flow technique. The reaction is first-order with respect to each of the reactants. From the distribution diagrams for the species DH(+)(2), DH and D(-) for DCPI and H(2)Q, HQ(-) and Q(2-) for sulphides or sulphites, a mechanism is proposed which suggests partial reactions of all possible combinations of the reacting species at any pH. An equation for calculation of the second-order reaction rate constants k at any pH is derived, which gives k as a function of [H(+)], the partial reaction rate constants and the dissociation constants of DCPI and H(2)S or H(2)SO(3). Values of the overall reaction rate constants over a wide pH-range have been determined, together with values of k for all possible partial reactions. For particular pH-values the second-order reaction rate constant was determined by four different methods. Mean values of k = 251 +/- 1 and 240 +/- 1 l.mole(-1).sec(-1) were obtained for pH 3.15 and 4.17, respectively, for the DCPI-Na(2)S reaction and k = 137 +/- 1, 127 +/- 1 and 136 +/- 1 l.mole(-1).sec(-1) for pH 2.02, 4.25 and 5.10, respectively, for the DCPI-Na(2)SO(3) reaction. From the slopes of the linear Arrhenius plots activation energies of 6.6 +/- 0.2 and 4.0 +/- 0.1 kcal/mole for the DCPI-Na(2)S and DCPI-Na(2)SO(3) reactions, respectively were calculated. The effect of ionic strength on the reactions supports the proposed mechanism.