Chemistry Unit, Research and Training School, Indian Statistical Institute, Calcutta-35 Received 4 February 1971 Adsorption isotherms of crystal violet, a triphenylmethane dye, on H-bentonite and H-kaolinite are of the Langmuir-type. No exact correspondence between the cation exchange capacities of the clays and the sorbed amount of dye is noticed. Both the clays take up dyes in excess of their exchange capacities. Adsorption of the whole unionised dye-molecules in the interlamellar spaces and a possible diznerisation of the dye on clay surfaces are suggested as an explanation. The exchangeability of the adsorbed dye with smaller quaternary organic cations is poor. The effect of increasing the alkyl chain-length of the quaternary cations up to four carbon atoms is found to be insignificant. The surface active quaternary cations are far more effective as desorbing agent. A maximum of 80% of kaolinite-bound dye and 30% of bentonite-bound dye are found to be displaced by cetylpyridinium ion. The ability of the cetyltrimethyl­ammonium and cetylpyridinium ion to open up the clay lattices considerably seems to help the interlamellar dyes to diffuse out. The detergent property of such ions is also operative.