Phosphine oxide is the inorganic compound with the formula H3PO.

Although stable as a dilute gas, liquid or solid samples are unstable.

Unlike many other compounds of the type POxHy, H3PO is rarely discussed and is not even mentioned in major sources on main group chemistry.

[1] H3PO has been detected by mass spectrometry as a reaction product of oxygen and phosphine,[2] by means of FT-IR in a phosphine-ozone reaction[3] Phosphine oxide has been claimed as the product of a reaction of phosphine with vanadium oxytrichloride as well as with chromyl chloride.

[4] 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.