Methacrylonitrile was copolymerized with p -methylstyrene in methyl ethyl ketone at 80-degrees-C initiated by azobisisobutyronitrile. Monomer reactivity ratios of methacrylonitrile and p-methylstyrene were found to be 0.205 and 0.377, respectively, using the Kelen-Tudos method. Triad fractions and monomer sequence lengths of three copolymers were determined from C-13-NMR spectra and were found to be in good agreement with those calculated from reactivity ratios.