In a recent study Sheng et al. (1995, A.I.Ch.E.J. 41 (10) 2306-2313) presented activity coefficients calculated by molecular simulation (MS) for athermal model polymer-solvent systems. Both activity coefficients of the monomeric solvent in the polymer (YT) and of the polymer in the solvent (7~) were calculated at infinite dilution conditions. The MS data cover a broad range of system asymmetry with respect to size in the area of oligomer-solvent mixtures (up to segment ratio 60/1 which corresponds to e.g. a system of n-heptane with nC46o). The MS results were compared with classical and recently proposed free-volume (FV) activity coefficient models in order to conclude on the suitability of the models for phase equilibrium calculations for asymmetric systems. On the basis of the work by Sheng et al., extrapolation methods for estimating the activity coefficient of a solvent and that of a polymer in real solvent-polymer systems are developed here. The so-obtained MS-based activity coefficients are compared with experimental data (in the case of solvent activities) and with the predictions of various activity coefficients models (in the case of polymer activities).