Protein separation mechanisms by a crossflow ultrafiltration membrane process are investigated using interfacial phenomena analysis of protein-protein and protein-membrane interactions. Varying operating conditions are used and membrane morphological parameters like hydrophilicity and hydrophobicity obtained from contact angle measurement. A single protein solution of bovine serum albumin (BSA) or ovalbumin (OV) or lysozyme (LY) is used in each filtration study case under various operating conditions, such as pH, ionic strength, and different polyethersulfone (PES) membranes with varying surface hydrophobicity and bydrophilicity. Under these conditions analyse of protein-membrane interfacial phenomena and protein separation filtration mechanisms are undertaken. Results indicate that electrostatic forces play an important role in protein deposition on the membrane surface and the rate of protein transmission across the membrane. Findings show that, at the proteins isoelectric point (pI), a hydrophobic membrane causes severe protein adsorption to the membrane surface, allowing a very small percentage of protein to be transported to the permeate side. (C) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.