The adsorption behavior of methylene blue (MB) on four fuller's earth (FE) samples of varying compositions was investigated using a spectrophotometric technique to obtain information on the color removal. The distribution coefficient (K-D) increased with an increase in the initial concentration (C-0) of the dye, attained a maximum value, and decreased again at higher initial concentrations. Dye solutions became colorless for a C-0 value corresponding to maximum K-D. A progressively increased flocculation behavior in the clay suspensions was observed and the maximum value of K-D corresponds to optimum flocculation of the clay. The K-D values were found to decrease exponentially after of the solution again became colored while the amount adsorbed increased with an increase in the initial concentration of MB. Only adsorption data obtained for this region could be defined by adsorption isotherm equations. The shifts of the C-0 values corresponding to K-D(max) toward higher concentrations were correlated with the composition of FE samples by using XRF, XRD patterns, and SEM images. The influence of temperature on MB adsorption was also studied and thermodynamic parameters were calculated. (C) 2003 Elsevier Science (USA). All rights reserved.