Adsorption of a cationic dye, methylene blue (MB), on the surface of montmorillonite leads to the molecular aggregation of dye cations, reflected by significant changes of dye optical properties. Montmorillonite samples, saturated with various inorganic cations (mono-, bi-, and trivalent, including those of transition metals), were used. Influence of the exchangeable cations on the MB aggregation was tested. Various properties of cations were considered (charge, diameter, acidity, hydration energies). Both direct and potential indirect effects of the cations were taken into account, such as salting-out effect, influence of the ions on solvent polarity, influence on swelling, colloid propel-ties of montmorillonite dispersions, cation hydration properties, hydrolysis, and interaction of the cations with the clay surface. The spectra of MB in dispersions of montmorillonite saturated with NH4+, K+, Rb+, and Cs+ were significantly different from those of other reaction systems. Direct association between large monovalent cations and basal oxygen atoms of silicate probably leads to a partial fixation of the cations, which affects the ion exchange reaction and dye aggregation. Thus, the presence of large monovalent cations leads to the formation of fewer ordered H-aggregates in favor of monomers and aggregates of lower size. In these cases, dye species absorbing light of low energies also appeared in significant amounts and were assigned to J-aggregates, characterized by a head-to-tail intermolecular association. (C) 2003 Elsevier Inc. All rights reserved.