Thallium(I) oxide is the inorganic compound of thallium and oxygen with the formula Tl2O in which thallium is in its +1 oxidation state. It is black and produces a basic yellow solution of thallium(I) hydroxide (TlOH) when dissolved in water. It is formed by heating solid TlOH or Tl2CO3 in the absence of air. Thallium oxide is used to make special high refractive index glass. Thallium oxide is a component of several high temperature superconductors. Thallium(I) oxide reacts with acids to make thallium(I) salts. Thallium(I) oxide, like all thallium compounds, is highly toxic.