The terpolymerization propagation kinetics of p-methoxystyrene, styrene and methyl methacrylate were investigated using pulsed-laser polymerization. In this work we used parameters obtained from the constituent copolymerization reactions to predict the propagation rate coefficients expected on the basis of an implicit penultimate unit model. In other words the model prediction was made with no adjustable parameters. We showed that this model could predict the experimental propagation rate coefficient values in this terpolymerization. This provides strong evidence for the validity of the implicit penultimate unit model.