Measurement of translational diffusion coefficients by laser light scattering has been proposed as a convenient, rapid, and accurate method for characterization of macromolecular species in solution. We have determined diffusion coefficients at infinite dilution in various solutions of a series of narrow molecular weight polystyrene samples and found that the data are consistent with only one of two studies reported in the literature. The concentration dependence of the diffusion coefficient for molecules of different particle weight is investigated, and the infinite dilution values are used to access the unperturbed hydrodynamic dimensions and the excluded volume. The diffusion data are compared with the corresponding intrinsic viscosity values and related to the particle weight. Finally a simple method for evaluating macromolecular polydispersity is described and applied to two polystyrene samples with broad molecular weight distribution.