Gravimetric sorption experiments were conducted to measure the diffusivity and solubility of hydrocarbon solvents in ethylene/styrene interpolymers over wide solvent concentration and temperature ranges. Even though the styrene content varied considerably, the solubility of the investigated solvents in different samples of the interpolymer were similar. Diffusivity correlations applying the Vrentas-Duda free-volume theory fit the experimental data well. The diffusivity decreases with increasing copolymer styrene content. Inverse gas chromatography experiments at low temperatures show the typical bend in the diffusivity data when passing through the glass transition temperature of the polymer. The diffusivity below the T_g is essentially constant.