Ternary diffusion coefficients for two concentrations of mixtures of two monodisperse polystyrenes of different molecular weight in toluene were measured at 25.00° with the Gouy interferometer. This system was chosen as a model of a polydisperse polymer suitable for experimental study. The results show large multicomponent effects for diffusion coefficients based on concentration gradient. The coefficients measured satisfy the Onsager reciprocal relations within experimental error. They are in qualitative agreement with those predicted from extensions of the hydrodynamic model.