Thermal oxidation of trace quantities of curium hydride (CmH2–3) has been reported to produce a volatile form of CmO2 and the volatile trioxide CmO3, one of the two known examples of the very rare +6 state for curium. Another observed species was reported to behave similarly to a supposed plutonium tetroxide and was tentatively characterized as CmO4, with curium in the extremely rare +8 state; however, new experiments seem to indicate that CmO4 does not exist, and have cast doubt on the existence of PuO4 as well.