A new method which proposes the use of a steady state CSTR is presented for the determination of rate parameters in a copolymerization system. The approach does not limit the conversion levels in the reactor and allows precise estimation of reactivity ratios. In the presence of gel effect, the suggested approach additionally reveals the cumulative variations of termination rate processes with extent of conversion. Such an a priori method which enables estimation of rate parameters in the high conversion region has not been available in the literature previously. The method proposed is illustrated using copolymerization of styrene–methyl methacrylate as a model system.