Processes involving high pressure polymer-solvent systems are becoming more common. Accurate modeling of the phase behavior is critical to the design engineer. One of the few thermo-dynamic models which can treat such systems is the Group-Contribution, Lattice-Fluid equation of state (GCLF-EoS). In this chapter a review of the model is presented. Different types of applications of the model (prediction/correlation) are presented. Examples of the model applied to high pressure systems are provided.