The solubility and diffusion coefficients of various solvents in ethylene-propylene copolymers with different ethylene contents were measured over a temperature range of 30-200 degrees C using capillary column inverse gas chromatography and gravimetric sorption. The value of the Flory-Huggins parameter decreased with an increase in temperature. The infinitely dilute diffusion coefficients obtained by the two methods were consistent with each other. The data indicate that the diffusion coefficients and solubility parameters are independent of the ethylene content of the copolymers. The results obtained from the sorption technique were extrapolated using a free-volume theory to the limit of zero solvent concentration, and the free-volume model was used to interpret the diffusion data. Analysis of the diffusion data indicated that the inclusion of an activation energy in the free-volume model does not significantly improve the correlation of the diffusion data.