Polymeric membranes were prepared from homogeneous solutions of two different mixtures: polysulfone (PSF)/dimethylformamide (DMF) and PSF/polyvinylpyrrolidone (PVP)/DMF. PSF solutions of 15 wt%, after cast on glass plates, were solidified either by direct immersion into distilled water or by 5-hr exposure under an environment of 72.5% relative humidity followed by immersion in a water bath. The prepared membranes are compared to each other on morphological and functional characteristics. In the vapor exposure process, the demixing rate of a cast solution film is enhanced with the addition of a polymer additive, PVP, in a casting solution. However, the enhancement of demixing rate did not result in an increase in water permeability or a significant morphology variation of the prepared membranes. On the contrary, when a cast film is coagulated by direct immersion into a water bath, the PVP in a solution film worked as a significant flux enhancer. It is concluded that at the specific concentration of 15 wt% polymer, PVP addition induces an increase in demixing rate of a casting solution, which results in an increase in permeability of the prepared membranes, only if the coagulation of a cast film is carried out in a nonequilibrium state.