Plutonium hydride is a non-stoichiometric chemical compound with the formula PuH2+x. It is one of two characterized hydrides of plutonium; the other is PuH3. PuH2+x is non-stoichiometric with a composition range of PuH2 – PuH2.7. Metastable stoichiometries with an excess of hydrogen (PuH2.7 – PuH3) can also be formed. PuH2 has a cubic structure. It is readily formed from the elements at 1 atmosphere at 100–200°C: When the stoichiometry is close to PuH2 it has a silver appearance, but gets blacker as the hydrogen content increases, additionally the color change is associated with a reduction in conductivity. Studies of the reaction of plutonium metal with moist air at 200–350°C showed the presence of cubic plutonium hydride on the surface along with Pu2O3, PuO2 and a higher oxide identified by X-ray diffraction and X-ray photoelectron spectroscopy as the mixed-valence phase PuIV3−xPuVIxO6+x. Investigation of the reaction performed without heating suggests that the reaction of Pu metal and moist air the production of PuO2 and a higher oxide along with adsorbed hydrogen, which catalytically combines with O2 to form water. Like the free metal, plutonium dihydride is pyrophoric. On the surface of hydrided plutonium, it acts as a catalyst for the oxidation of the metal with consumption of both O2 and N2 from air.