Nonsolvent induced phase inversion has been widely used to prepare micro-porous to almost dense membranes. In this study, variations in the thermodynamic and kinetic properties of polyethersulfone (PES) casting solution caused by polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) additives were investigated. The thermodynamic variation was realized to enhance demixing rate of solvent and nonsolvent in the casting solution, while the rheological variation induced the opposite trend, delayed demixing. The morphology of the final membranes was affected by trade-off between thermodynamic enhancement and kinetic hindrance. The effect of thermodynamic enhancement and kinetic hindrance on membrane properties was tracked through two simple non-dimensional parameters. By changing the concentration and molecular weight (MW) of polymeric additives, these two parameters were controlled, and a wide range of structures ranging from finger-like to sponge-like voids were prepared. Lowering the demixing rate resulted in denser membranes with sponge-like structure, whereas at higher demixing rates, membranes with large finger-like pores were produced. Synthesized membranes had molecular weight cut-off (MWCO) between 0.1 and 500 kDa, which demonstrates the principal effect of polymeric additives on morphology of polymeric membranes. (C) 2013 Elsevier B.V. All rights reserved.