The bulk copolymerizations of acrylonitrile (AN) with vinyl acetate VAc initiated by azobisisobutyronitrile (AIBN) and the suspension copolymerization of AN with VAc and sodium methallylsulfonate (SMAS) in water with a Na2S2O5-Na2ClO3, redox initiator system at 65 degreesC, were investigated. The copolymer compositions were determined by H-1-NMR. The reactivity ratios (gammas) for the two copolymerization systems were determined analytically, based on Mayo-Lewis equation, by fitting the calculated curves with the experimental data. The ys for the AN and VAc bulk copolymerization were found to be gamma (12) = 2.85 and gamma (21) = 0.11. The values of the apparent gammas for the suspension copolymerization of AN, VAc, and SMAS were as follows: gamma (12) = 3.58, gamma (21) = 0.39, gamma (13) = 1.45, gamma (31) = 0, gamma (23) = 0.92, and the rate constant ratio R-3 k(31)/k(32) = 0.04. A simulated result produced with the obtained gammas agreed fairly well with experimental data of bulk copolymerization in a batch reactor. The apparent gammas obtained were also successfully used to analyze the results of suspension polymerization in a continuous pilot reactor.