Polyvinylpyrrolidone free and thus more sustainable polyethersulfone ultrafiltration membrane were successfully synthesized by the incorporation of high amounts inorganic nanoparticles into the polymeric matrix. The effect of the filler content on particle aggregation, membrane characteristics and membrane performance was examined. An interplay of both rheological and thermodynamical effects on phase separation was observed and caused an optimum in membrane permeability. This optimum was depending on the nanoparticle type. Best results were obtained with a PES membrane containing 50 wt% titania nanoparticles. The optimized membranes were compared with earlier-made PSf membranes with micron-sized ZrO2 particles, but the so-called nano-effect was less pronounced than expected. (C) 2014 Elsevier B.V. All rights reserved