A two-phase kinetic model is proposed for the emulsion grafting copolymerization of styrene and acrylonitrile in the presence of polybutadiene seed latex particles. The experimental results observed by conducting the polymerizations at 50 degrees C are compared with those predicted by the proposed kinetic model and confirm that the model is valid for predicting the rate of this emulsion grafting copolymerization. In the model predictions, thermodynamic equilibrium equations proposed in our previous papers are used for calculating the monomer concentrations in poly(styrene-co-acrylonitrile) domains and continuous polybutadiene matrix in acrylonitrilebutadienestyrene terpolymer particles as emulsion grafting copolymerization proceeds. In addition, it is shown that the model is also applicable to predict grafting efficiency, copolymer composition of free and grafted chains.