The Gibbs free energy, the enthalpy, and the entropy of mixing of ethanol with the polymer poly(vinylpyrrolidone) (PVP) and with its low molecular weight analog, N-ethylpyrrolidone (NEP) were calculated. The calculation of the free energy of mixing was achieved with the thermodynamic model for hydrogen-bonded polymer systems developed by Painter, Coleman, and Graf. This model, based on the use of an association model, gives the free energy of mixing as a function of the Flory-Huggins interaction parameter, the composition of the mixture, and the association equilibrium constants. The self-association of the ethanol molecules was described by two equilibrium constants, one for the formation of dimers and one for the formation of multimers. The equilibrium constants of interassociation of PVP or NEP with ethanol were determined from the quantitative analysis of NEP/ethanol and PVP/ethanol FTIR spectra at various temperatures and compositions. The values of the equilibrium constants were then used to calculate the theoretical Gibbs free energy of mixing as a function of the composition. The enthalpic and entropic contributions to mixing were compared for the NEP/ethanal and PVP/ethanol mixtures.