The ability of the Sanchez‐Lacombe equation of state to predict the solubility of small penetrants in rubbery polymer matrices, over a wide range of ambient pressures, is examined critically. The solute chemical potential and, in turn, the resulting predicted isotherms are found to depend strongly on solute equation of state parameter values and on the mixing parameter for polymer‐penetrant interactions. The isotherms are less sensitive to polymer equation of state parameters. In the sorption of organic vapors into polymers, the model does not describe well the sorption isotherms at high values of penetrant activity. The model appears to describe sorption isotherms most accurately when both polymer and penetrant equation of state parameters are determined from pure component properties, in the same temperature and pressure range as the sorption data, and when a mixing parameter is used to adjust the fit of the model to the data. © 1993 John Wiley & Sons, Inc.