In the present study a comprehensive mathematical model for diffusion-controlled free-radical copolymerization reactions is developed. Termination and propagation rate constants as well as initiator efficiency are expressed in terms of a reaction-limited term and a diffusion-limited one. The contribution of the latter term to the apparent rate constants is described in terms of the diffusion coefficients of the corresponding species (e.g., polymer chains, monomers, primary radicals), calculated using the generalized free-volume theory of Vrentas and Duda for a ternary system, and an effective reaction radius. The predictive capabilities of the present model are demonstrated by simulating three free-radical polymerization systems, namely, the bulk copolymerizations of styrene-methyl methacrylate, styrene - acrylonitrile, and p-methylstyrene-methyl methacrylate. It is shown that the model predictions are in excellent agreement with experimental data on monomer conversion, average molecular weights, and copolymer composition, reported in the open literature under various experimental conditions.