The group contribution-Flory equation of state (GC-Flory EoS) is applied to the prediction of the phase behavior of monodisperse polymer/single solvent systems. The model is capable of predicting with satisfactory accuracy the most common types of phase diagrams typical of liquid-liquid equilibria of polymer solutions (i.e., phase diagrams of the UCST, LCST, combined (UCST + LCST), and hourglass types). Combinatorial effects derived from differences in size, shape, and structure of the polymer and the solvent molecules strongly influence the phase behavior of the systems, but the type of a phase diagram of a specific polymer/solvent system is primarily governed by the nature of the molecular energy interactions. The GC-Flory EoS predicts the significant effect of the free-volume contributions at high temperatures, which is in very good agreement with the nature of the liquid-phase separation at temperatures near the gas-liquid critical temperature of the solvent, where the highly expanded state of the solvent leads to LCST behavior.