Micellar enhanced ultrafiltration (MEUF) is used to remove acid dye from aqueous solution. Cetyl(hexadecyl) pyridinium chloride (CPC) and eosin (molecular weight 691.86) have been taken as cationic surfactant and acid dye, respectively. An organic polyamide membrane of molecular weight cut-off 1000 is used in unstirred batch experiments. The effects of surfactant-to-dye concentration ratio in the feed and the transmembrane pressure drop on permeate flux and observed retention of the dye have been studied as a function of the operating time. The retention of dye without using surfactant is only 10% at a typical feed dye concentration of 10 x 10(-3) kg/m(3). Whereas under the same operating pressure (276 kPa), retention increases up to 73.4% using surfactant micelles. The maximum retention of eosin is obtained at a surfactant-to-eosin ratio of 2000. Free surfactants molecules present in the permeate are then recovered by a two-step chemical treatment procedure. In the first step, surfactant is precipitated by potassium iodide and in the second step, the surfactant is recovered from the precipitate by addition of cupric chloride. In the first step, 1: 1 molar ratio of potassium iodide to CPC results in about 90% precipitation of the surfactant. In the second step, 5 : 1 molar ratio of cupric chloride-to-cetyl pyridinium iodide (CPI, precipitated form of CPC) leads to about 90% recovery of the surfactant. (C) 2003 Elsevier B.V. All rights reserved.