Gas sorption and dilation isotherms of polymers have been classified and described by a model incorporating the concepts of plasticization and isothermal glass transition. The model applies to both glassy state and rubbery state of the polymer-penetrant systems. It is also shown that the dilation isotherms can be characterized by a parameter ƒ, which reflects the plasticization ability of the penetrant species adsorbed in the unrelaxed volume of the polymers. Physical interpretations have been given to parameters of the model. The work may provide some new knowledge of glassy transition, and interactions between molecules of penetrants and polymers.