The effects of the copolymerization system on the monomer reactivity ratios for a copolymer of acrylonitrile with N-vinylpyrrolidone was studied for contrast. The values of the monomer reactivity ratios were calculated by the Kelen-Tudos method. The reactivity ratios in the aqueous suspension polymerization system were similar to those in the solution polymerization system at polymerization conversions of less than 6% [reactivity ratio of acrylonitrile (r(AN)) = 0.438 +/-0.015, reactivity ratio of N-vinylpyrrolidone (r(N-VP)) = 2.28 +/- 0.02]. As conversion of more than 12%, the changes in the monomer reactivity ratios became less prominent (r(AN) = 0.651, r(N-VP) = 1.98). In the water-rich reaction medium [H2O/dimethylsulfoxide (DMSO) > 60/40], the monomer reactivity ratios were approximately equivalent to those in the aqueous suspension polymerization system. In the DMSO-rich reaction medium (DMSO/H2O > 60/40), the reactivity ratios were similar to those in the solution polymerization system. With an increase in the polarity of the solvent, the values of the reaction ratios both decreased. The values of the reaction ratios gradually tended toward I with increasing copolymerization temperature. (C) 2003 Wiley Periodicals, Inc.