The dependence of propagation and termination rate coefficients on conversion in free-radical polymerization is described by a purely kinetic model which considers contributions of segmental diffusion, translational diffusion, and reaction diffusion to termination and which also takes diffusion-control of the propagation step into account. The kinetic scheme provides a satisfactory representation and interpretation of the termination rate coefficient measured in an extended range of conversion for ethylene, butyl acrylate, and methyl methacrylate polymerization. Reaction diffusion is of primary importance for termination at moderate and high conversion, which leads to fairly simple expressions for kinetic parameters even at conversions where the solution viscosity exceeds the pure monomer viscosity by several orders of magnitude.