In this work the chromium exchange mechanism in zeolite NaY was studied. The breakthrough data were determined in an up-flow fixed bed at 30 degrees C using three bed heights: 1.5, 3.0 and 4.5 cm. It was seen that the bed height influenced the maximum chromium uptake as well as the mass transfer zone due to changes in pH during the ion exchange process. Changes in pH generated different chromium species with particular difficulties in diffusing towards the exchange sites of the zeolite. The linear driving force (LDF) model for breakthrough curves fitted well to the experimental data and the estimated overall mass transfer coefficient also changed with bed height, which is a consequence of different chromium speciation in the ion exchange mechanism. The axial dispersion coefficient can be considered as an average of 8.25 x 10(2), 1.04 x 10(2) and 3.36 x 10(2) cm(2)/min for the bed heights of 1.5, 3.0 and 4.5 cm, respectively. (c) 2005 Elsevier B.V All rights reserved.