Thallium(III) oxide, also known as thallic oxide, is a chemical compound of thallium and oxygen. It occurs in nature as the rare mineral avicennite. Its structure is related to that of Mn<sub>2</sub>O<sub>3</sub> which has a bixbyite like structure. Tl<sub>2</sub>O<sub>3</sub> is metallic with high conductivity and is a degenerate n-type semiconductor which may have potential use in solar cells. A method of producing Tl<sub>2</sub>O<sub>3</sub> by MOCVD is known. Any practical use of thallium(III) oxide will always have to take account of thallium's poisonous nature. Contact with moisture and acids may form poisonous thallium compounds.
It is produced by the reaction of thallium with oxygen or hydrogen peroxide in an alkaline thallium(I) solution. Alternatively, it can be created by the oxidation of thallium(I) nitrate by chlorine in an aqueous potassium hydroxide solution.