The reactivity ratios for the copolymerization of methyl, propyl, butyl, heptyl and cetyl methacrylates with vinyl acetate have been determined. The values obtained are r₁=22.21±0.89 r₂=0.072±0.026 for methyl ester copolymer, and r₁= 73.3±7.4, 62.1±3.8, 60.4±0.4, 68.3±3.2 ;r₂=0.186±0.038, 0.127±0.015, 0.271±0.039, 0.135±0.055 for the other copolymers respectively. Since these values remain almost unchanged when alkyl group increased from three to sixteen carbon atoms, obviously the size of alkyl group in the ester has little influence upon the monomer reactivity in polymerization.