Arsenic tetroxide is an inorganic compound with the chemical formula As<sub>2</sub>O<sub>4</sub>, containing As(III) and As(V), As<sup>III</sup>As<sup>V</sup>O<sub>4</sub>.
It can be synthesized in an autoclave via the following reaction:
It adopts a layer structure, and the coordination geometry of As(III) is triangular pyramid while As(V) is tetrahedral.