A counter current multi-stage fluidized bed ion exchanger is developed to study mass transfer during the continuous removal of dissolved anions from wastewater using commercially available resin. OH ion is used as an example in the study. A higher removal efficiency in the multi-stage fluidized bed than in a single-stage fixed and fluidized bed is demonstrated. The experiment shows progressive fluidization on a stage, smooth flow of resin across the stage and transfer of resin from one stage to the other. In each stage of the fabricated four-stage perspex made column, a downspout has been provided to facilitate the downward flow of resin on to the next stage, while water flows counter currently upward through the mesh of the stage. In addition, provision has been made to adjust the downcomer height on the stage without disturbing the operation with the aid of the rack and pinion arrangement. The experimental variables in the multi-stage column operated under steady state includes the flow rates of water and resin, feed concentration, stage height and the number of stages. A mathematical model is also developed for determining the key parameters that affect the overall mass transfer in the multi-stage continuous counter current column. In general, number of stages and diffusional resistance on the resin side control the extent of separation in the column. (c) 2005 Elsevier B.V. All rights reserved.