Polyethylene (PE) membranes containing controlled pore sizes and porosities were fabricated using a polytetramethylene glycol (PTMG) mixture with paraffin as a novel diluent via a thermally induced phase separation (TIPS) process. PTMG could not be used as a diluent alone regardless of its molecular weight because it does not form a single phase mixture with PE over the entire temperature range. However, PTMG and paraffin mixtures, which exhibited upper critical solution temperature (UCST) type phase behavior, formed miscible blends with PE at elevated temperatures. The phase separation temperature of the PE//(paraffin/PTMG) ternary mixture increased with increasing PTMG content in the diluent mixture when PE content was fixed. At a fixed blend composition, the phase separation temperature of the PTMG/paraffin mixture increased as the molecular weight of the PTMG decreased, while that of the PE//(paraffin/PTMG) ternary mixture increased with increasing PTMG molecular weight. The observed phase behavior was attributed to the intermolecular association behavior of PTMG. The phase separation temperature in the PE//(paraffin/PTMG) ternary blend could be increased up to the evaporation temperature of paraffin by controlling the PTMG content or its molecular weight. As a consequence, a PE membrane containing controlled pores and porosities was able to be fabricated using a diluent mixture containing PTMG and paraffin. (c) 2012 Elsevier B.V. All rights reserved.