The effective diffusion coefficients of dextran and polyethylene glycol in tr track-etched polycarbonate membranes were measured using membranes with nominal pore sizes of 0.03 mu m, 0.05 mu m, and 0.1 mu m. Experiments were performed using narrow-size range fractions of each polymer. When the Stokes-Einstein radius was used to describe solute size, the observed diffusivities for both polymers agreed closely and were larger than values predicted for rigid spheres, as well as for linear polymers when only steric interactions with the pore wall are assumed. These observations cannot be explained by considering electrostatic interactions between the solute and the pore wall, or solute adsorption on the pore wall. The experimentally measured diffusion coefficients agreed well with a model that treats the polymeric solutes as rigid spheres and includes van der Waals attractive interactions between the solute and the pore; wall.