Several commercial polymeric ultrafiltration membranes were screened for their performance with aqueous ethanol solutions. The method of conditioning the membrane has a major effect on solvent flux, membrane integrity and their pressure ratings. Gradual solvent exchange with successively higher concentrations increased in small doses appears to work best with completely miscible solvents such as those studied here (ethanol-water mixtures). Rapid solvent exchange between water and high concentrations of alcohol disrupts the polymer matrix in many cases. The Darcy model was used to correlate the data and it indicated that viscosity differences of the ethanol solutions could account for part of the variations in solvent flux with some membranes. Exposure to organic solvents significantly reduces the pressure rating of the membranes. Several membranes that provided acceptable rejection of ethanol-soluble proteins at low pressures (138 kPa, 20 psi) lose its properties at higher pressures (413 kPa, 60 psi) if conditioned incorrectly, and vice versa.