In this study, the flux behavior of an-inclined flat-membrane cross-flow ultrafiltration was investigated. The experiment was conducted using a cellulose ester membrane (10,000 MWCO) and dextran T500 as the test solution. The membrane inclination included 0degrees (flow above membrane), 90degrees (vertical membrane), and 180degrees (flow below membrane). Experimental results showed that raising the cross-flow velocity increased the permeate flux and the time required to reach a steady state was also shortened. The effect of inclination on the flux enhancement is significant when the conventional ultrafiltration is operated at a low liquid flow rate. In two-phase flow ultrafiltration, the introduction of gas enhances the permeate flux even at a high liquid flow rate and the influence of gas sparging is particularly prominent as the membrane is installed at 180degrees orientation. Under the present operating conditions, the filtration flux is maximum at 180degrees and minimum at 0degrees inclination. (C) 2004 Elsevier B.V. All rights reserved.