The induction period observed in the cis‐RuCl2(DMSO)4 catalyzed oxidation of thioethers (S) to sulfoxides by N‐methylmorpholine N‐oxide (NMO) is attributed to the rate of formation of the active species viz. RuCl2(DMSO)3S. Cyclic voltammetric studies indicate formation of the RuIV = 0 species. Based on the variable orders in NMO and first order in each of the other reactants, a mechanism is proposed where the active species generated in situ reacts with NMO in a slow step to form RuIV oxo complex which then decomposes to give the product. Copyright © 1990 John Wiley & Sons, Inc.