In spiral wound membrane (SWM) modules, spacers are essential for keeping the membrane leaves apart. Feed side spacers also enhance flux by promoting eddies, but the disadvantage of spacers is that they increase the pressure loss along the flow channel. The effect of the filament diameter. mesh length and arrangement of spacer filaments was investigated experimentally for ultrafiltration (UF) of dextran and whey solutions. For conventional 2-layer feed spacers, a reduction of the mesh length first increases flux and then flux decreases beyond some optimum mesh length while pressure loss increases continuously with a decrease of the mesh length. An advanced 3-layer spacer (A3LS) is developed with superior mass transfer characteristics and less fouling propensity when compared with conventional 2-layer spacers. The 3-layer spacer improves flux without covering additional membrane area by filaments adjacent to the membrane wall under both fouling and non-fouling conditions. However, the current AMS design also increases the pressure loss. Under fouling conditions the A3LS has more than 20% higher flux when compared with the 2-layer spacer. (C) 2003 Elsevier B.V. All rights reserved.