Aqueoussuspensions of different Na-smectite type clay minerals were exchanged with methylene blue (Mb) and analyzed by visible spectroscopy. The spectra show bands of two types of monomers, protonated Mb, Mb-dimers, and higher aggregates. Their relative importance and the bandwidth was found to depend on parameters such as the particle morphology, the degree of dispersion, and the extent and location of the layer charge of the smectite. This can be qualitatively explained by the relative importance of three types of interactions, Mb-surface, H(2)O-surface, and Mb-Mb interactions. For hectorite and laponite at small loadings, H(2)O-surface interactions are dominant. Mb-islands are formed with a characteristic monomer absorption at 653 nm. Monomers at the surface absorb at 670 nm as found in barasym and saponite. Wyoming bentonite takes an intermediate position. As the loading increases Mb-Mb and Mb-surface interactions become dominant, giving rise to monomers absorbing at 670 nm, dimers, and higher aggregates. The bandwidths of the absorption bands reflect the structure of the clay particle associates. Copyright 1999 Academic Press.