The group-contribution, lattice-fluid equation of state (GCLF-EoS), has been previously shown to predict accurately vapor solubilities in polymers. The method has been extended to the predictions of carbon dioxide-polymer binary systems. Applying the GCLF-EoS to binary systems requires the characterization of all the groups in the system and estimating the molecular binary interaction parameter from the group interaction parameters. The group parameters for pure carbon dioxide and the group interaction parameters of carbon dioxide with other organic groups commonly found in polymers were estimated. A new mixing rule for the calculation of the molecular interaction parameter is proposed and compared with the one developed originally by Lee and Danner. Results show that the new mixing rule as applied to the sorption of carbon dioxide in polymers and copolymers gives good predictions in large ranges of pressure and temperature.