Flux decline in the dead-end ultrafiltration (UF) of binary solutions of bovine serum albumin (BSA, p14.7, MW 67,000) and hemoglobin (Hb, pI 7.1, MW 68,000) with polyethersulfone (PES, 100 kDa) and polyacrylonitrile (PAN, 100 kDa) membranes was studied. Key factors affecting the flux decline including solution fill (6.00-7.50), total protein concentration (1.5-9.0 mu M), transmembrane pressure (TMP, 10-50 psi), and ionic strength (0.01-0.1 M) were systematically investigated. It was shown that the blocking filtration law could satisfactorily analyze the flux decline behavior. A simplified exponentially time-dependent model was adopted to describe the dynamics of flux decline during UF process, and used to determine the adequate tinge for membrane cleaning. The mechanism of membrane fouling was also analyzed by blocking filtration law, in which the standard blocking always dominated at the early stage in such UF processes. The fouling mechanism strongly depended on hydrophobic characteristics of the membranes, concentration and charge of the proteins, as well as pH and ionic strength of the solutions. (C) 2008 Elsevier B.V. All rights reserved.