AB ST R ACT : The adsorption of methylene blue (MB) on montmorillonite from acid solutions has not yet been studied in detail. In this paper the adsorption of this dye on Ca/SAz montmorillonite from 0.32, 0.80 and 1.60 M HCl solutions at ambient temperature and after 2 hours leaching was carefully studied and evaluated. Under the above conditions the MB/SAz intercalates formed are practically insoluble. In contrast to the adsorption of methylene blue on montmorillonite from aqueous solutions, the adsorption from acid solutions consists of two adsorption waves. The intercalated samples from the first and second waves were studied using X-ray powder diffraction (XRD), FTIR, Vis spectroscopy, and scanning electron microscopy (SEM) equipped with a microprobe system. The adsorption of MB in the first wave is attributed to ion exchange of the dye cationic species (MB + , MB 2 2+ , H-aggregates, MBH 2+ and H + ) for the original interlayer cations of montmorillonite. The percentages of MBH 2+ and H + based on total adsorbed MB were estimated for varying HCl concentrations. The second adsorption wave is linked to the precipitation of MB species on the outer MB/SAz surface.