An approximate equation was developed that permits calculation of the solvent tracer diffusion coefficient in a homogeneous swollen membrane from the measured hydraulic permeability coefficient. This relation was applied to data for 28 polymer–solvent systems that included 15 different organic solvents and 5 hydrocarbon polymer networks whose equilibrium swellings ranged from 16.1 to 91.5% polymer on a volume basis. The calculated tracer diffusion coefficient divided by the pure solvent self-diffusion coefficient for these systems formed a unique correlation when plotted versus the polymer volume fraction in the membrane. This relation agreed well with tracer diffusion coefficient data in the literature for the benzene–natural rubber system measured by radioactive tagging. Discussion centers on the evidence for the validity of the equation developed and the role of hydrodynamics on diffusion in swollen membranes.