Liquid velocity and gas holdup were measured in a 690L pilot-scale rectangular airlift contactor, divided into aerated and nonaerated sections by two vertical rectangular polyester textile membranes. Horizontal flow through the porous membranes complicates flow fields to such an extent that conventional models cannot predict thoroughly velocity profile in the contactor. Experimental data obtained from laser doppler velocimetry (LDV) method were compared with numerical finite volume method (FVM), in order to determine the velocity profile in the downcomer zone of the contactor. In formulating the data, a realistic set of boundary conditions was implemented in the numerical method, resulted in satisfactory agreement between theory and experiment. The effects of the distance between the rectangular membranes and air bubble size on the velocity profile were also studied. (c) 2006 American Institute of Chemical Engineers.