To understand the effects of the phase behavior of the diluent mixture on the microstructure of polyethylene (PE) membranes, PE membranes were prepared from PE blends with various diluent mixtures including dioctyl phthalate (DOP)/polytetramethylene glycol (PTMG), soybean oil (SBO)/PTMG, and liquid paraffin (LP)/PTMG via a thermally induced phase separation (TIPS) process. The DOP/PTMG mixtures were always miscible, while the SBO/PTMG and LP/PTMG mixtures exhibited upper critical solution temperature (UCST)-type phase behavior. The PE/diluent-mixture blends showed UCST-type phase behavior, and their phase separation temperatures increased with increasing PTMG content in the diluent mixture. Just above the PE crystallization temperature, the PE-rich phase formed from the PE//(LP/PTMG) blend contained a larger amount of diluent than that formed from the PE//(DOP/PTMG) or PE//(SBO/TMPG) blends. The number of pores formed from the PE//(DOP/PTMG) or PE//(SBO/TMPG) blends by liquid liquid phase separation was much greater than that formed from the PE//(LP/PTMG) blend. As a result, the porosities of PE membranes fabricated using the DOP/PTMG or SBO/PTMG mixtures were higher than those fabricated using the LP/TMPG mixtures.