Abstract Heterophase radical copolymerization of acrylonitrile and methacrylic acid initiated by K 2S 2O 8 in the presence of (C 4H 9) 4NBr has been studied. From the composite microstructure of the copolymers the authors determined the apparent relative activities of the monomers on heterophase copolymerization in different conditions as a function of the specifics of initiation. A phase analysis is made of the reaction systems providing further information on the interphase contacts on copolymerization. The influence of pH, the ionic strength of the aqueous phase and the area of the interphase surface on the parameters of the process was studied. It is shown that the acts of initiation and propagation of polymer chains on copolymerization initiated by interphase transfer come about in the interphase layer.