The copolymerization of methyl methacrylate (MMA) and vinyl acetate (VAc) under high oxygen pressure was investigated. Copolyperoxides of various compositions were synthesized by the free-radical-initiated oxidative copolymerization of NMIA and VAc monomers. The copolyperoxide compositions obtained from H-1 and C-13 NMR spectra were used for determining the reactivity ratios of the monomers. The reactivity ratios indicated a larger proportion of MMA units statistically placed in the copolyperoxides. A theoretical analysis based on semiempirical AM1 calculations was performed to support the reactivity ratios. NMR studies showed irregularities in the copolyperoxide chain due to the cleavage reactions of the propagating peroxide radical. Thermal analyses of the copolyperoxides by differential scanning calorimetry gave evidence for the presence of alternating peroxide units in the copolyperoxide chain, The activation energies of thermal degradation suggested that degradation was controlled by the dissociation of the peroxide (-O-O-) bond in the backbone of the copolyperoxide chain.