A comprehensive mathematical model is developed to describe the kinetics and molecular and compositional developments in a free-radical terpolymerization batch reactor. This model is based on a fairly general kinetic mechanism, including chain-transfer and terminal double-bond reactions. We take into account the effects of diffusion-controlled phenomena (i.e., gel, glass, and cage effects) on polymerization kinetics by extending our previous model on diffusion-controlled reactions to terpolymerization systems. Triple moments for the live and dead trivariate chain-length terpolymer composition distributions are introduced to describe the molecular and compositional developments in the terpolymerization system. The predictive capabilities of this model are demonstrated by simulation of the free-radical, bulk terpolymerization. of butyl acrylate/methyl methacrylate/vinyl acetate under different experimental conditions. It is shown that the model predictions are in good agreement with experimental data on monomer conversion, average,molecular weights, and terpolymer composition, as reported by Dube and Penlidis. (C) 2003 Wiley Periodicals, Inc.