Abstract Ab α′-cobalt-substituted nickel hydroxide has been prepared from a β(II)-substituted nickel hydroxide by an oxidation to a γ-type phase followed by a reduction. This α′ Co 3+ -hydroxide crystallizes in the hexagonal system with a P 3-type hydroxyl packing ( a = 3.1 A, c = 23.5 A). β(II)- and α′-cobalted phases have been characterized by chemical analysis, TGA, u.v.-visible and i.r. spectroscopies. In both materials cobalt ions remain in the trivalent state. In α′ Co 3+ -hydroxide, the excess of positive charge is compensated by carbonate anions which are either inserted between the M(OH) 2 slabs, or adsorbed on the particles. The infrared study shows that inserted anions are totally hydrogen bonded with intercalated water molecules ( D 3 h symmetry), while the adsorbed ions are linked to the nickel or cobalt ions in substitution of hydroxyl groups. In β Co 3+ -hydroxide, the charge compensation due to the presence of trivalent cobalt ions is provided by chemisorbed carbonate anions and by a proton deficiency in the sheets. The bonding between the inserted carbonate anions and the Co 3+ cations increases the slab cohesion; as a result the α′ Co 3+ -phase ( x Co ⩾ 0.2) is stable indefinitely in KOH medium.