Polystyrene/toluene mutual-diffusion coefficients have been measured as a function of temperature in the limit of infinite solvent dilution. The solvent to polymer jump size unit ratio (xi) was determined from the Vrentas-Duda free-volume diffusion model for polymer self-diffusion and is in excellent agreement with values evaluated from solvent self- and binary mutual-diffusion coefficient data. Comparison of the free-volume model to a version of the Kirkwood-Riseman theory, modified for diffusion at infinite dilution under non-theta conditions, suggests that xi follows the temperature dependence of the root-mean-squared end-to-end distance of the polystyrene and can be estimated without the use of diffusion data. This result establishes an explicit theoretical coupling between thermodynamic interactions, Brownian dynamics, and free-volume theory.