The volume changes caused by the sorption of H2O, Ar, N-2, CO2, CH4 and Acetone in Bisphenol-A polycarbonate and of CO2 in different substituted polycarbonates and in Kapton were measured in a dilatometer at room temperature. The partial molar volumes of the small molecules are much smaller than the ones obtained in the liquid or rubbery state of polymers. It is a special feature of the glassy state that the par-tial molar volume increases as concentration increases. Both findings are explained by a model developed recently, where the volume of the site occupied by small molecules is related via elastic distortions to the solution energy into this site. Assuming a spherical shape of site volumes and a Gaussian distribution of the volumes yields an average value of 33 to 38 Angstrom(3)/site and a width of about 10 Angstrom(3). These quantities vary in a systematic manner with the glass transition temperature and the nature of side groups of the polymer. The results are compared with values calculated from lifetimes of o-positronium and values of the d-spacings from X-ray.