The effect of solute concentration on hindered diffusion was investigated by measuring diffusion rates of dextran and polyethylene glycol (PEG) in track-etched polycarbonate membranes at concentrations up to 60 mg/mL. Results were interpreted using a model that incorporates a first-order virial coefficient to describe the concentration dependence of the equilibrium partition coefficient, as well as the intrapore diffusivity. For dextran, both virial coefficients were positive, indicating that the effective diffusivity increases with solute concentration. Measured values of the virial coefficient for equilibrium partitioning agreed with a model that includes attractive van der Waals interactions between dextran and the pore wall. For PEG, both virial coefficients were negative, indicating that the diffusivity decreases as solute concentration increases. These observations may be due to chain entanglement or aggregation that occurs with increasing PEG concentration.