Sepiolite and zeolite, highly porous minerals, are becoming widely used as alternative materials in areas where sorptive, catalytic and rheological applications are required. High ion exchange capacity and high surface areas and more importantly their relatively cheap prices make them attractive adsorbents. In this study, the adsorption mechanism of three reactive azo dyes (Reactive Black 5, Red 239 and Yellow 176) by two natural mezoporous minerals has been examined in order to identify the ability of these minerals to remove coloured textile dyes from wastewaters. For this purpose, a series of batch adsorption tests were carried out as a function of solids concentration, time and dye concentration using natural and modified sepiolites and zeolites. The adsorption results indicate that both natural sepiolite and zeolite have limited adsorption capacities of the reactive dyes but are substantially improved upon modifying their surfaces with quaternary amines. The mechanism of adsorption process is elucidated on the basis of experimental data. (C) 2003 Elsevier Ltd. All rights reserved.