Copolymerization of acrylonitrile with vinyl acetate in the presence of a complexer—ethylaluminium dichloride—has been studied. It is shown that irrespective of the composition of the comonomer mixture for a molar fraction of acrylonitrile <0.6 an alternating copolymer of equimolar composition forms. The possibility of chain propagation both through attachment of the free monomers to the growing active centres and by attachment of the donor-acceptor complex incorporating both comonomers is discussed. A number of parameters has been obtained characterizing the reaction of chain propagation in the system indicated and it is shown that chain propagation operates by a mixed mechanism.