The influence of an admixture of polyvinylpyrrolidone (PVP) with different molecular weights on the morphology and properties of polyethersulfone (PES) membranes prepared via water vapor-induced phase inversion followed by immersion in water has been studied. Membrane structure alteration by PVP addition to the casting solution has been observed by means of SEM. The contact angle on monolithic PES films varies from 78A degrees to 54A degrees depending on the PVP content. By Fourier-transform IR spectroscopy, it has been revealed that an increase in molecular weight of PVP leads to an increase in its residual content in the membranes. It has been shown that the addition of PVP with a molecular weight close to that of PES hardly has a significant effect on the permeability and tensile strength of the membranes regardless of the PVP content. However, the introduction of high-molecular-weight PVP leads to a decrease in the pore size and permeability of the membranes as a result of change in viscosity of the solution.