the phase separation phenomena in polymer solutions were studied via phase diagrams of polystyrene-solvent systems determined by differential scanning calorimetry and turbidimetry method. it was demonstrated that the critical temperatures found in this study and the theta temperatures from the research literature, show a linear relationship as expected from the Shultz-Flory equation. Thermally induced phase separation (TIPS) process was employed to prepare microporous membranes, and the phase separation mechanisms were studied based on the phase diagram for the system. It has been shown that if polymer concentration is close to the critical point and phase separation is initiated in the unstable region, resulting in a spinodal decomposition mechanism, the membrane produced was well interconnected with highly uniform pore sizes and possesses mechanical strength. In contrast, the characteristic membrane morphology associated with polymer concentration being below the critical concentration and phase separation being initiated in the metastable region, resulting in a nucleation and growth mechanism, was poorly interconnected, stringy and/or beady structure which is mechanically fragile.