Poly(vinyl butyral) (PVB)-based ultrafiltration membranes were prepared by immersion-precipitation in water coagulation bath. The effects of different factors, such as PVB concentration, scraper clearance, water temperature and especially solvent nature on the performance of final membranes were extensively investigated. The toplayer formation mechanism of the PVB membranes prepared in four different PVB-solvent-water systems was discussed. By comparison with poly(ether sulfone) (PES), the hydrophilicity of PVB was also studied. Experimental results showed that PVB-based ultrafiltration membranes with an asymmetric structure were successfully prepared by the observations of scanning electron microscope (SEM). The formation of the toplayer of the PVB membranes above-mentioned from different solvents was the result of the combination of the equilibrium thermodynamics and membrane formation kinetics, whereas, the sublayer of those membranes was dominated by the diffusion rate of solvent-non solvent. The permeability and separation performance of the final PVB membranes could be adjusted under different preparation conditions. PVB-PES blends were the nonmiscible systems, and the increase in mass ratio of PVB in the components decreased the surface contact angle of the resulting PVB-PES composite membranes from 81.0 degrees to 76.6 degrees. In phase diagram, the width of the liquid-liquid homogeneous region of the PES-DMAC-water system was very close to but slightly smaller than that of the PVB-DMAC-water system, which indicated that PVB was a little more hydrophilic than PES while in solution. (c) 2005 Elsevier B.V. All rights reserved.