Specific reactions between montmorillonite and Co in low concentrations were studied in the presence of excess CaCl2. Two forms of bound Co were distinguished. One form, characterized as being slowly dissociable, appears to be bound in a monolayer by chemisorption and will exchange with Cu++, Zn++, or other Co++ and to a lesser extent with ions of other transition metals but not with Ca++, Mg++, or NH4+. A second form of Co did not dissociate and could not be extracted from the mineral with 2.5% acetic acid. This fraction, which was not characterized, is thought to either enter the crystal lattice or be occluded in the precipitation of another phase.