Pure hexagonally layered α-Co(OH)2 powder is prepared in large quantity by precipitation method under mild conditions. Subsequently, anion-exchange reaction and aging treatment in alkaline medium are performed to find their influence on the microstructure and supercapacitive properties of α-Co(OH)2. Various inorganic anions are successfully intercalated into the interlayer gallery of α-Co(OH)2 with the morphology and structure reserved. While the α-Co(OH)2 transforms into other cobalt compounds by aging treatment 1MolL−1 KOH solution at room temperature. The effects of anion-exchange on the supercapacitive properties are evaluated by cyclic voltammetry and galvanostatic charge–discharge tests. The α-Co(OH)2 exhibits a high specific capacitance, good rate capability and improved stability after intercalation with nitrate and sulfate. The morphology and structure of α-Co(OH)2 after 2000 continuous charge–discharge tests in 1MolL−1 KOH electrolyte are also well maintained, indicating its excellent stability in charge–discharge tests. In contrary, the phase transitions from α-Co(OH)2 to β-form and other cobalt phases lead to a dramatic decrease in the specific capacitance, from 763.4Fg−1 to 25.9Fg−1.