In this study, the effects of some factors on the adsorption of a basic dye (methylene blue) on bentonite and sepiolite samples were studied. These factors are cation (Na+ and Ca2+) saturation of the samples and pH and ionic strength of the dye solution. The adsorption data were found to conform to the Langmuir equation within the concentration range studied and Langmuir constants were determined for each of the samples. The adsorption capacities of the samples were found to increase with cation saturation. Changes in the pH of the dye solution had no significant influence on the adsorption capacity and adsorption capacities were found to decrease with increasing ionic strength. The maximum sorption capacity of methylene blue exceeded the cation exchange capacity of bentonite and sepiolite. (C) 2004 Published by Elsevier Inc.