The affect of competing surfactant molecules, n-decyltrimethylammonium bromide (C10TAB) and n-hexadecyltrimethylammonium bromide (C16TAB), on the adsorption equilibrium between methylene blue (MB) and Na+-montmorillonite has been studied using visible spectrophotometry. The MB competed effectively with both these surfactants and also tetramethylamntonium ions, which were used as a model for the surfactant head group. At loadings up to 50% of the CEC of the clay, all of the MB was associated with the clay or clay/surfactant system. Evidence for the presence of monomer, MB+, dimer, (MB+)2, trimer, (MB+)3, and protonated monomer, MBH2+, was obtained in each case, with the trimeric form dominating at loadings >5% of the CEC. The spectra of adsorbed MB reflected the difference in chain length between C10TAB and C16TAB and indicated that a significant amount of monomeric, MB+, was solubilized in the C16TAB surfactant clusters on the clay surface. Pre-adsorbed MB was not displaced by added C16TAB surfactant.