The ability of some antural Mexican clays to sorb radioactive Co from aqueous solution and then desorb it has been measured. The clay cations seem to be exchanged with cations present in the solution. It was observed that the amount of sorbed Co2+ depends nearly linearly on the Na++K+ content in the montmorillonites. It was found that the crystallinity was not altered after Co2+ sorption. Crystallinity of clays, before and after cobalt exchange, was determined by X-ray diffraction.