A G(E) model is used for the correlation and prediction of partial miscibility in polymer solutions that exhibit both upper critical solution temperature and lower critical solution temperature as well as hourglass-type phase separation in some cases. The model employs a combinatorial/free-volume term and the residual term of UNIQUAC, for which interaction parameters between the solvent molecule and the repeating unit (segment) of the polymer are determined. The obtained parameters are then used to predict the effect of the polymer molecular weight on the solvent/polymer partial miscibility. The model is also successfully applied in the prediction of the pressure effect on mutual solubility by incorporating it through the free volumes of the components. Finally the prediction of excess properties using the proposed model is investigated.