Recent results concerning the prediction of thermodynamic properties of solutes in polymers are presented. In particular, the computation of vapor-liquid and gas-liquid equilibria (i.e., liquid and gas solubilities) in different polymers and partition coefficients between the polymer and a solvent phase are addressed. Calculations have been carried out using COSMO-RS theory which combines quantum-chemical calculations with efficient statistical thermodynamics for intermolecular interactions. Predictions for vapor liquid equilibria and for partition coefficients have been improved by incorporation of polymer-specific entropic contributions due to free volume effects. It is demonstrated that a high predictive accuracy is obtained if the polymer is sufficiently characterized by its composition, density, and crystallinity. The approach is currently limited to gaseous and liquid solutes and to linear, i.e. non-cross-linked polymers without any significant swelling.