Microporous membranes were prepared by the thermally induced phase separation (TIPS) process of a ternary solution of HDPE, ditrydecylphthalate and hexadecane. The equilibrium phase diagram calculations were carried out in terms of a pseudo-binary system, considering the solvent mixture to be one component, to show the effect of the interaction parameter on the phase diagram. The experimental results showed that by varying the composition of the solvent pair, i.e., by varying the interaction parameter systematically, the phase diagrams can be controlled successfully. Liquid-liquid phase separation was obtained also below the melting point by controlling the composition of the solvent mixture. It is also shown that the membrane morphology can be controlled with different solvent compositions when the cooling conditions are kept constant.