The group-contribution lattice-fluid equation of state was applied to gas-polymer binary systems. New groups were characterized in terms of their parameters and their interaction with several other organic groups. A new mixing rule for the calculation of the molecular interaction parameter is proposed and compared with one developed originally by Lee and Danner (1996). Results show that the new mixing rule gives better predictions in most cases. For some binaries, involving methane for example, both mixing rules fail to represent correctly the sorption isotherms. An attempt is made to try to understand the model limitations.