Polysulfone (PSF) membranes were prepared via the phase inversion process from casting solutions consisting of PSF. n-methylpyrrolidone (NMP), and polyvinylpyrrolidone (PVP). The PVP as a non-solvent additive reduces the miscibility of casting solutions with non-solvent water, which causes thermodynamic enhancement for phase separation. and also induces the increase in solution viscosity, which causes kinetic hindrance against phase separation. In demixing of cast solutions through the vapor exposure, the thermodynamic enhancement and the kinetic hindrance works simultaneously, showing a trade-off relationship against one another. At low concentration (5 wt.%) of PVP. the variation in thermodynamic property controls the demixing process to result in the enhanced phase separation of the solution. With further increment of PVP. however, the solution demixing is delayed and the kinetic hindrance due to viscosity buildup overwhelms the thermodynamic factor: the enhancement in thermodynamic immiscibility could not contribute to the increase in demixing rate. When prepared via immersion process, the membranes show the similar trend in the water permeability as the phase separation data in the vapor exposure. That is, at low PVP concentration of 5 wt.%, the flux increases with the enhancement in the phase separation. but it drops with further increase of PVP in the casting solution rather than improved.