A gas chromatographic method was used in order to measure vapor-liquid equilibria for solvent (1)-polymer (2) systems in which the polymers were polystyrene, poly (a-methyl) styrene and the solvents were benzene, toluene, cyclohexane, methylisobutylketone, ethylacetate, and vinylacetate, The activity coefficients of solvents for solvent (1)-polymer (2) systems were measured at infinite dilution and the modified ASOG (Analytical Solution of Group) model was suggested to describe vapor-liquid equilibria of those systems within a range of temperatures 423.15K through 498.15K, The model consists of the original ASOG and the free volume term, An external degree of free dom in the free volume term empirically became to a C-1 = alpha + beta/T as a function of temperature. Each term in the modified ASOG model is based on the weight fraction, The external degree of freedom in the model was estimated by experimental data within a range of temperatures. As a result of doing it, the infinite dilution activity coefficients calculated were agreed with the experimental data within an error of 0.1 %.