Purpose: The interaction of methylene blue (MB) with Na-saturated clay from Maghnia deposit in aqueous suspension was investigated using visible absorption spectroscopy. Methods: The effect of the optical properties of dyes in aqueous solutions and adsorbed onto Na-saturated clay was assessed in terms of the absorption bands maxima and the removal rate as a function of pH. Results: The adsorption of MB dye onto Na-saturated clay surface leads to the methacromatic effect in the absorption spectrum of the dye. Formation of MB dimers and higher agglomerates depended sensitively on the potential of the layer charge density. MB agglomerates occurred predominantly in the Na-saturated clay with high charge density, even with low charge density. The redistribution of MB molecules over the surface of Na-saturated clay of lower charge density (high acidic medium) leads to a reduction of the dye aggregation, accompanied by a bathochromic shift of the band maximum of MB dye to the longer wavelengths. Conclusion: The methachromatic effect was analyzed by the R=A(MB+)/A(MB+)n parameter, which decreases with the charge density, owing to the increase in the pH solution. To explain the relationship between the layer charge density and the adsorption, so- colled model 2-pKa of surface complexation (SCM) was developed.