Some features of radical ternary copolymerization of maleic anhydride (MA)-styrene (St)-acrylonitrile (AN) and n-butyl methacrylate (BMA)St-AN acceptor-donor-acceptor monomer systems have been revealed. The terpolymer compositions and kinetics of copolymerizations were studied in the initial and high conversion stages. The considerable divergence in the copolymer compositions was observed when a strong acceptor MA monomer was substituted with BMA having comparatively low acceptor character in the ternary system studied. Obtained results show that terpolymerization proceeded mainly through "complex" mechanism in the state of near binary copolymerization of St...MA (or BMA) and AN...St complexes only in the chosen ratios of complexed monomers. The terpolymers synthesized have high thermal stabilities (295-325 degrees C), which is explained by possible intermolecular fragmentation of AN-units through cyclization and crosslinking reactions during thermotreatment in the isothermal heating conditions.