A method is proposed to predict solvent activities in amorphous polymer solutions. The method relies on two main assumptions : (i) the configurational entropic term of the excess Gibbs energy of mixing is adequately represented by the classical Flory-Huggins relation; (ii) the enthalpic term arising from the attractive interactions between the solvent molecules and the segments which constitute the polymer can be described by using the NRTL equation. To calculate the binary interaction parameters of the NRTL equation, the same strategy as previously adopted is followed to predict vapor-liquid equilibria of non-polymeric mixtures. Accordingly, literature experimental data are subdivided into the following classes of mixtures : non-polar polymers in non-polar solvents non-polar polymers in polar solvents polar polymers in non-polar solvents polar polymers in polar solvents For each of the above classes, simple rules are proposed which link the binary energy parameters of the NRTL equations to the difference between the Hildebrand solubility parameters of the polymer and the solvent. The proposed method is applied to over forty binary mixtures for which reliable data are found in the literature. A new procedure to calculate the Hildebrand solubility parameter of the polymer is also reported.