Phosphine oxide is the inorganic compound with the formula H<sub>3</sub>PO. Although stable as a dilute gas, liquid or solid samples are unstable. Unlike many other compounds of the type PO<sub>x</sub>H<sub>y</sub>, H<sub>3</sub>PO is rarely discussed and is not even mentioned in major sources on main group chemistry.
H<sub>3</sub>PO has been detected by mass spectrometry as a reaction product of oxygen and phosphine, by means of FT-IR in a phosphine-ozone reaction
Phosphine oxide has been claimed as the product of a reaction of phosphine with vanadium oxytrichloride as well as with chromyl chloride. The product was obtained by matrix isolation. It has also been reported relatively stable in a water-ethanol solution by electrochemical oxidation of white phosphorus, where it slowly disproportionates into phosphine and hypophosphorous acid.
Phosphine oxide is reported as an intermediate in the room-temperature polymerization of phosphine and nitric oxide to solid P<sub>x</sub>H<sub>y</sub>.