Methyl vinyl ketone (MVK) was first used to successfully copolymerize with acrylonitrile (AN). This was achieved with azobisisobutyronitrile as the initiator. The kinetics of the copolymerization of AN with MVK were investigated in a H2O/dimethyl sulfoxide (DMSO) mixture between 50 and 70 degrees C under N-2 atmosphere. The rate of copolymerization was measured. The kinetic equation of the copolymerization system was obtained, and the overall activation energy for the copolymerization system was deter-mined. The values of the monomer apparent reactivity ratios were calculated by the Kelen-Tuclos method. In a DMSO-rich reaction medium (DMSO/H2O > 80/20), the monomer apparent reactivity ratios were similar to those in the solution polymerization system. (c) 2005 Wiley Periodicals, Inc.