Cobalt(III) oxide is the inorganic compound with the formula of Co<sub>2</sub>O<sub>3</sub>. Although only two oxides of cobalt are well characterized, CoO and Co<sub>3</sub>O<sub>4</sub>, procedures claiming to give Co<sub>2</sub>O<sub>3</sub> have been described. The treatment of Co(II) salts such as cobalt(II) sulfate with an aqueous solution of sodium hypochlorite (also known as bleach) gives a black solid:
Some formulations of the catalyst hopcalite contain "Co<sub>2</sub>O<sub>3</sub>".
Some studies have been unable to synthesize the compound, and report that it is theoretically unstable.
It is soluble in cold diluted sulfuric acid and produces Co<sub>2</sub>[SO<sub>4</sub>]<sub>3</sub>, which is blue in aqueous solution.
Cobalt(III) ion is a strong oxidizer in acidic solution, its standard electrode potential is +1.84V in this situation.