Free-radical polymerization in supercritical carbon dioxide is a heterogeneous process that has recently attracted increased research and industrial interest. In the present paper, a comprehensive mathematical model is developed for the dispersion polymerization of vinyl monomers in supercritical CO2. A detailed kinetic mechanism is employed to describe the molecular weight development in the two-phase polymerization system. The concentrations of the various reacting species in the two phases as well as the pressure variation during polymerization are calculated using the Sanchez-Lacombe equation of state. The predictive capabilities of the present model are demonstrated by comparison of the theoretical predictions with experimental results on monomer conversion, total pressure, and molecular weight averages.