The rate of chemical-initiated solution copolymerization of acrylonitrile and styrene in toluene as solvent was studied theoretically and experimentally with a limiting conversion of about 50%. Taking into consideration industrial operation, the polymerization was carried out under an azeotropic composition of AN:St = 25:75 by weight, and a new kinetic model is proposed for estimating the conversion and the rate of polymerization as well as the molecular weight distribution over the entire reaction course. It is shown that the experimental data of conversion and the rate of reaction, as well as average molecular weight, can be successfully correlated to limiting conversion by this model if the effect of change in the initiator efficiency during the reaction on the rate of reaction is taken into consideration. It was found that the effective initiator efficiency can be expressed as f = 0.0956([A0] + [S0]). Some important kinetic constants as well as model parameters are obtained by simulation of this model, which agree with the experimental results. Certain transfer constants obtained by this simulation model are kf12 = 30 × kf11 and kf21 = 5 × kf22. Within the limiting conversion studied, no significant autoacceleration is observed. It is therefore not necessary to include this effect in the model.