The self-diffusion of styrene, polystyrene, and 2,2'-azobisisobutyronitrile has been determined in the bulk polymerization of styrene with pulsed-gradient spin-echo nuclear magnetic resonance at 25 degreesC. Data on small molecules are discussed with respect to recent diffusion models. They can fit self-diffusion coefficient data of small molecules in dilute or semidilute polymer solutions; in concentrated solutions, however, there is a breakdown. A semiempirical model based on scaling laws is used to describe the self-diffusion of styrene and 2,2'-azobisisobutyronitrile over the whole range of concentrations studied. The dependence of the polystyrene self-diffusion coefficient on the polymer concentration is described with a stretched exponential function, D = D-o exp(-alphac(v)), where a depends on the molecular weight of the polymer and V depends on the kind of solvent. (C) 2003 Wiley Periodicals, Inc.