Inverse gas chromatography was used to measure activity coefficients of solvents at infinite dilution over a temperature range from 373.15K to 423.15K for monodisperse polystyrene-hydrocarbon systems. Polystyrene has average molecular weight 2.89×10⁵. Solvents include six aliphatic hydrocarbons such as cyclohexane, methylethyl-ketone, n-hexane, carbon tetrachloride, acetonitrile and acetone and three aromatic ones as benzene, toluene and p-xylene. The measured data of activity coefficients of solvents were predicted by UNIFAC-FV model. We, also, pres-ented a modified UNIFAC-FV model by empirically assigning to it a temperature-dependent C parameter in the free volume contribution of the model. Fitted results demonstrated that the modification of the model fitted the experiments better than the original one.