Ultrafiltration (UF) membranes based on poly(vinyl chloride) and poly(vinyl pyrrolidone) blends were prepared by the phase inversion method, and the factors governing membrane properties were investigated. The membranes were characterized by scanning electron microscopy and atomic force microscopy. The fouling characteristics of the membranes were determined by UF of aqueous solutions of bovine serum albumin (BSA) over a pH range of 2-9 and varying salt concentrations. The maximum adsorption of the protein on the membrane surface occurred near the isoelectric point (pI 4.8) of BSA, and the presence of the salts increased the fouling of the membrane. The results can be explained in terms of the nature of the membrane polymer and the effect of different ionic environments on the permeability of the deposited protein layer. The net charge on the BSA molecules appears to be a dominant factor in determining the flux of water through the blend membranes. The UF flux is correlated by a model based on the membrane resistance, adsorbed protein resistance, and time dependent resistance of the concentration polarization layer near the membrane surface. The zeta potentials of the membranes were also determined before and after UF to characterize the surface potential of the membrane.