An isopiestic apparatus using the quartz crystal balance principle has been constructed for vapor sorption measurements on polymers. Isothermal solubilities of benzene, toluene and chloroform in poly(styrene) were measured in the concentrated polymer regime. Benzene results were obtained at 323.15 K, Toluene and chloroform results were obtained at 298.15 K and at 323.15 K. Solvent activities were represented with a modified Flory-Huggins activity model and compared with available literature data with favorable results. Comparisons were also made with predictions of three free volume activity models described in the literature. Details of the applicability and reliability of the quartz crystal balance for these sorption measurements are considered and discussed.