Aluminium(II) oxide or aluminium monoxide is a compound of aluminium and oxygen with the chemical formula AlO. It has been detected in the gas phase after explosion of aluminized grenades in the upper atmosphere and in stellar absorption spectra.
Aluminium(II) oxide is one of the aluminium oxides (the most common is aluminium oxide Al<sub>2</sub>O<sub>3</sub>), as it was the rare example of an aluminium(II) compound since aluminium usually exists in its +3 oxidation state.
Aluminium monoxide is an unstable compound, due to it easily reacting with oxygen to become aluminium(III) oxide: