Phosphorus monoxide is an unstable radical inorganic compound with molecular formula PO.

It was detected in the circumstellar shell of VY Canis Majoris and in the star forming region catalogued as AFGL 5142.

The compound has been found to have been initially produced in star-forming regions, and speculated to be carried by interstellar comets throughout outer space, including to the early Earth.

In 1894 W. N. Hartley was the first to report an observation of ultraviolet emission from a phosphorus compound, that was later expanded on by Geuter.

The source of the spectral lines and bands were known to be related to phosphorus, but the exact nature was unknown.

[5] Phosphorus monoxide is believed to be the most abundant phosphorus-containing molecule found in interstellar clouds.

VY CMa was studied using the Submillimeter Telescope (SMT) of the Arizona Radio Observatory (ARO).

[9] On Earth, phosphorus monoxide can be prepared for study by spraying phosphoric acid into a flame.

Because commercial acetylene gas contains some phosphine, an oxy-acetylene flame will have weak PO emission bands in its spectrum also.

[12] Phosphorus monoxide can act as a ligand on transition elements such as molybdenum, ruthenium and osmium.

[5] The dipole moment of the molecule is 1.88 D. The phosphorus atom has a slight positive charge calculated as 0.35 of the electron.