The aqueous polymerization of MMA was studied kinetically at 40° C using low concentrations of Py-SO2complex as initiator. For [Py-SO2] < 2 × 10−2mol/L, Rp∞ [PY-SO2]0.5[M]1.5, and for [Py-SO2] > 2 × 10−2mol/L, Rp∞ [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-SO2complex and monomer molecules. For [Py-SO2] < 2 × 10−2mol/L, the polymerization is characterized by bimolecular termination. Above this [Py-SO2], chain termination by a degradative initiator transfer process assumes prominence.