A group-contribution method is presented for the estimation of solvent activities in polymers solutions. The method is an extension of the UNIFAC, which was originally derived for mixtures of ordinary liquids. The fundamental assumption is the additivity of the contributions made by the functional groups which constitute the compounds in the mixtures. The extension to mixtures with polymers is attractive because as show by T. Oishi, J.M. Prausnitz, Ind. Eng. Chem. Process Des. Dev. 17 (1978) 333, the solvent activities in various polymers solutions can be assumed with an uncertainty of no more than 10%, with the advantage that for the UNIFAC method no experimental data are required. However, this method has been mainly proven in non polar systems. In this paper, the method is extended to cases in which polymers and/or solvents have polar groups and when the solid phase is a copolymer with a random distribution. To calculate the influence of polar group, with stronger interactions, the solubility parameters concept is introduced. Also, a correction factor is used to take into account the distribution of repetitive units in the copolymer. Experimental results obtained by a gravimetric method were fitted with the proposed model. The polymers used were polydimethyisiloxane (PDMS) and poly(ethylene-co-vinyl acetate) (EVA) while the solvents were ethanol and n-hexane. The results fit the data very well for polar-polar system, within the range of temperature investigated. However, the equilibrium sorption data for the system n-hexane-EVA cannot be properly fitted with the UNIFAC model, mainly because the effect of vinyl acetate groups are rejecting n-hexane and its influence overcome its own boundaries in the copolymer structure.