Extensive but separate studies have been conducted to focus on utilization of swelling clays to remove cationic dyes from aqueous solution and to investigate the feasibility and applicability of using methylene blue (MB) adsorption for cation exchange capacity (CEC) and specific surface area (SSA) determination. This research aims at elucidating the mechanism of MB adsorption on low-charge montmorillonite in order to better understand the principles behind MB removal using swelling clays and to validate the practice of using MB for CEC and SSA determination. Stoichiometric desorption of exchangeable cations from the clays accompanying MB adsorption as well as the close match between the MB adsorption capacity and the CEC of the clays confirmed cation exchange as the most important mechanism for MB removal. XRD and TG-DTG analyses revealed interlayer adsorption, thus, intercalation of MB molecules. FTIR analyses suggested that hydrogen bonding may not play a major role in MB adsorption. The results confirmed that the charge density, rather than the SSA was the limiting factor for MB adsorption. For the treatment of wastewater containing cationic dyes, swelling clays with a high CEC value would result in a greater MB removal from water. (C) 2011 Elsevier B.V. All rights reserved.