Interaction of methylene blue (MB) with reduced charge Li-montmorillonites (RCM) in the aqueous suspensions was investigated using visible absorption spectroscopy. Dye cation agglomeration and protonation at the clay surface depended very sensitively on layer charge density. With increasing layer charge reduction, the content of agglomerates of MB cations decreased in favor of monomers and the protonated form of MB. Lower negative charge density on the clay basal surface induces a greater distance between neighboring MB cations sorbed at the clay surfaces, which suppresses dye agglomeration. Since each form of MB absorbs visible light at a different wavelength, different layer charge densities induce different colors of the resulting clay-dye suspension. Therefore, visible spectra of MB-smectite suspensions may be a simple but extremely sensitive method for the detection of layer charge density of smectites.