Partition of acrylonitrile(AN)between aqueous phase and oil phase and its effect on the copolymer composition in AN／St suspension polymerization were investigated．The result shows that the composition of copolymer in suspension polymerization differs from thatin bulk polymerization at the same monomer feed ratio．It is because the AN／St monmer ratio in the oil phase is lower than the fed one due to the significant phase partition of AN between aqueous phase and oil phase．In order to predict accurately and then control the copolymer composition in AN／St suspension polymerization

a model for calculating AN／St copolymer composition was proposed on considering phase partition of AN between the two phases．The results calculated by the model were in agreement with the experimental data．The real reactivity ratios in oil phase used in the model were the same as that in the bulk polymerization

but the apparent reactivity ratios in suspension polymerization varied with the water／oil ratio significantly．