The relationship between the aspect ratio (plate length L/plate width w) and the performance of membrane extraction systems in cross-flow parallel-plate membrane modules for a given set of operating conditions and system variables, has been delineated. The performance was described as the rate of solute extraction per unit power input. It was found that the optimal aspect ratio increases with the decrease of the flow rate in phase a flowing from x = 0 through x = L, but decreases with the decrease of the flow rate in phase b flowing from y = 0 through y = w, for enhancing the fluid velocity in the phase where the flow rate is lower, resulting in increased mass transfer coefficient. (C) 2002 Elsevier Science B.V. All rights reserved.