Abstract: Is this article, a new mathematical procedure for calculating the rate constants(k2/k1, k1)in 2-step series-parallel second-order reactions is presented. Under specific conditions, when k2/k1 is equal to one of 17 specific values and initial molar ratio of the reactants lies between the lower and upper boundary values, a set of exact equations for the determination of k1 are derived, in other cases, approximate equations or approximation method are proposed. All the exact and approximate equations can be covered by a equation. This procedure can be extended to derive multi-step series-parallel reactions. Finally this procedure is applied to the kinetic study of sulfonation of polysulfone and satisfactory results are obtained.