In this work, composition data and propagation rate coefficients for the copolymerizations of p-chlorostyrene with styrene and with p-methoxystyrene at 40°C were obtained. It was found that the terminal model provided an adequate simultaneous description of the copolymer composition and propagation kinetics in the p-chlorostyrene with p-methoxystyrene system, though not in the p-chlorostyrene with styrene system. For both of these systems it could be shown that, even given the uncertainties in the estimated monomer and radical reactivity ratios, it was clear that the monomer (r1r2) and radical (s1s2) reactivity ratio products were significantly different to each other (i.e. r1r2 ≠ s1s2)- a result which is inconsistent with the radical stabilization model. The implications of this result for the implicit and explicit penultimate models are discussed.