It is an ionic compound, with trivalent cobalt cations Co3+ and hydroxyl anions OH−.

The brownish-black form is a stable solid and can be prepared by reaction of water solutions of cobalt(II) chloride and sodium hydroxide, followed by oxidation with ozone.

[1] The green form, formerly thought to be cobalt(II) peroxide, apparently requires carbon dioxide as a catalyst.

It can be prepared by adding hydrogen peroxide to a solution of cobalt(II) chloride in 96% ethanol at –30 to –35°C, then adding a 15% solution of sodium carbonate in water with intense stirring.

The resulting dark green powder is fairly stable at liquid nitrogen temperature, but at room temperature it turns dark brown within a few days.