Silicon monoxide is the chemical compound with the formula SiO where silicon is present in the oxidation state +2. In the vapour phase it is a diatomic molecule. It has been detected in stellar objects and it has been described as the most common oxide of silicon in the universe.
When SiO gas is cooled rapidly, it condenses to form a brown/black polymeric glassy material, (SiO)_n, which is available commercially and used to deposit films of SiO. Glassy (SiO)_n is air- and moisture-sensitive. Its surface readily oxidizes in air at room temperature, giving an SiO₂ surface layer that protects the material from further oxidation. However, (SiO)_n irreversibly disproportionates into SiO₂ and Si in a few hours between 400 and 800°C, and very rapidly between 1,000 and 1,440°C, although the reaction does not go to completion.