The influençe of SnCl4 as modifier on the copolymerization of vinyl acetate (VAc) with acry- lonitrile (AİN) was studied in bulk using azobisisobutyronitrile as an initiator at 65°C. The kine- tic results shovv that the overall copolymerization rates increase with increasing the concentra- tion of SnCl4 in the reaction medium. In addition, higher relative rates were obtained in the medium containing higher content of AN in the monomer feed. The copolymerization parameters of the two monomers approach to each other with raising the molar ratio of SnCl4 in the system. Thus, the presence of SnCl4 increases the reactivity of VAc towards the copolymerization pro- cess and hence it raises the alternating charactcr of the resultant copolymers. Consequently, the copolymerization of VAc and AN in the presence of SnCl4 can be suggested as a multicom- ponent system, since the monomers can form complexes with SnCl4 as was observed from IR spectra and sixteen elementary polymerization reactions can be considered in the propagation process. The effect of modifier on the structure of the resultant VAc-AN copolymer as indicated fröin x-ray diffraction patterns are very useful in industry, since such copolymers are used for manufacture of membranes and for preparing fibrous activated carbon of high adsorption ca- pacity and strength.