A gas chromatographic method is used to measure the activities of solvents which have a range of finite concentrations in polymer solutions. Experimental data are presented for 8 binary solvent/polystyrene systems at 393.15 and 423.15 K. The polymer ASOG is proposed to be correlated with the activities of solvents in polymer solutions, and consists of the original ASOG and the empirical modification of free volume. For 8 binary solvent/polystyrene systems containing nonpolar and moderately polar solvents, the polymer ASOG was successful to correlate with experimental data within the range of 1% error.