In this study, we aimed to design PBI (poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole) blend membranes with a higher water flux by using a less amount of the expensive PBI material and by employing environmentally friendly ionic liquids as the solvent. Five commercially available polyimides and polyimide-amides were screened and P84 (BTDA-TDI/MDI, co-polyimide of 3,3',4,4'-benzophenone tetracarboxylic dianhydride and 80% methylphenylenediamine +20% methylenediamine) was chosen to blend with PBI because it formed miscible blends with PBI and interacted closely with the ionic liquid of 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc). An interesting interconnected network of the PBI/P84/[EMIM]OAc solution was evolved based on the highly charge-ordered characteristics of [EMIM]OAc. The blend solution displayed unusual rheological behavior: an initial shear thinning behavior under low shear rates followed by a Newtonian plateau. The incorporation of P84 in the blend system not only lowered the overall viscosity for easier membrane fabrication but also retarded the phase inversion process favorably to form a macrovoid-free morphology. PBI/P84 blend membranes were therefore fabricated for ultrafiltation via non-solvent induced phase inversion method. The effects of PBI/P84 composition and casting temperature on membrane morphology and separation performance were studied according to their phase inversion mechanisms. Compared to plain PBI ultrafiltration membranes, the newly developed PBI/P84 blend membranes exhibit an open cell structure and a reduced thickness which leads to a 50% higher pure water permeability and a larger pore diameter. (C) 2012 Elsevier Ltd. All rights reserved.