The pulsed-laser polymerization technique is used to determine the composition-averaged free-radical propagation rate coefficient (k(p,ter)) for terpolymerization of butyl methacrylate, butyl acrylate, and styrene between 60 and 120 degrees C. A significant deviation from terminal model predictions is observed for the ternary system, indicating that penultimate kinetics are important at these industrially relevant temperatures. The implicit penultimate propagation model, with all the coefficients taken from previous studies of the binary systems, provides a good prediction of the experimental k(p,ter) results.