The polymerization of methyl methacrylate (MMA) and acrylonitrile (AN) initiated by the oxygen-ascorbic acid (AA)-vanadyl ion (VO2+) system was studied at 40°C in dilute sulfuric acid medium. The rate of polymerization, Rp, was followed gravimetrically and found to be proportional to [MMA]1.3, [O2]0.6, and [VO2+]° at [VO2+] = 4–8 × 10−4Min the polymerization of MMA. In the polymerization of AN, R was dependent on [AN]1.5, [O2]0.5, and [VO2+]° at[VO2+] = 4–10 × 10−4M, while at [VO2+] > 10−3Mit was proportional to [AN]1.7, [O2]0.7, and [VO2+]0.6. A slight dependence of Rpon [AA] and [H2SO4] was observed in both polymerizations. An increase of ionic strength had an accelerating effect on Rp. An initial increase in Rpand then a decrease with temperature was noticed. A plausible reaction mechanism with initiation by the primary radical R produced from the dissociation of the O2-AA-VO2+complex and termination by mutual interaction with predominant oxidation of R by VO2+was found to be consistent for the observed kinetic results at all [VO2+] for MMA and for AN at low [VO2+]. Linear termination was found to be operative at high [VO2+] in the polymerization of AN. The chain length of the polymers was determined viscometrically.