Abstract: The adsorption of the cationic oxidized safranine S+ by a Na+, Ni2+ and Fe3+ montmorillonite has been studied with X-ray powder diffraction, u.v., visible and i.r. spectroscopy. In solution S+ may be protonated: S+, SH2+ and SH23+3+ have characteristic spectra in the 500–600 nm region where the clay structure does not absorb. In the Na+ as well as in the Ni2+ and Fe3+ clays, the adsorption of S+ is a cation exchange process accompanied by the protonation of the adsorbed dye such as variable concentrations of M+ (Na+, Ni2+ or Fe3+), S+ and SH2+ are simultaneously present. Protonation activity decreases from Fe3+ to Ni2+ and Na+, being the protonation site the amine group as shown by i.r. In the interlamellar space it seems that a SH2+.. S+ association exists that could be described as a sandwich structure 6.5 Å thick.