The kinetics of polymerization of acrylonitrile and methyl methacrylate initiated by the redox system 2,2′‐thiodiethanol/trichloroaquobipyridylmanganese(III) have been investigated in the temperature range 20–35°C in dilute sulfuric acid. The oxidation of the substrate in the absence of monomer has also been studied. A third‐order reaction involving the complex, the substrate, and H+ leads to the production of primary radicals which initiate polymerization of the monomers. The primary radicals are more reactive towards methyl methacrylate than towards acrylonitrile. The termination of polymerization with both monomers is by mutual interaction between the growing radicals. A suitable kinetic scheme has been proposed, and the rate constants evaluated. Copyright © 1982 John Wiley & Sons, Inc.