The aqueous polymerization of MMA was studied kinetically at 40° C using low concentrations of Py-S02 complex as initiator. For [Py-SO2] < 2 x 10-2 mol/L, Rpα [Py-SO2]0.5 [M]1.5 and for [Py-SO2] > 2 x 10-2 mol/L, Rpα[Py-SO2]0.0[M]1.08. Polymerization is considered to proceed by a radical mechanism. The radical generation or the initiation step is believed to proceed through equilibrium complexation between the Py-SO2 complex and monomer molecules. For [Py-SO2] < 2 x 10-2 mol/L, the polymerization is characterized by bimolecular termination. Above this [Py-SO2], chain termination by a degradative initiator transfer process assumes prominence. © 1981, Taylor & Francis Group, LLC. All rights reserved.