The diffusion coefficient of labeled poly(methyl methacrylate) (PMMA) has been measured by forced Rayleigh scattering from solutions containing labeled polymer, matrix PMMA, and toluene. Several combinations of the labeled and matrix polymer molecular weights were used. The matrix polymer concentration was varied from 0 to 40% by weight. Under the assumption that the rate of chain termination is controlled by the diffusion of the chain radicals, the diffusion data have been compared with available data for the rate coefficient of termination of PMMA radicals measured over the same polymer concentration range. The comparison shows that the distribution of chain radical molecular weight has a pronounced influence on the kinetics of termination.