After decades of believing in a very stable PuO 2 , suitable for final storage of nuclear waste, the existence of a higher oxide, PuO 2+x , was recently claimed. This would have far reaching consequences on the strategies of storage of Pu-based waste. Its formation therefore has been discussed controversially for several years.In this work, existence and stability of the higher oxide, PuO 2+x , has been probed by photoelectron spectroscopy study of PuO 2 exposed to atomic oxygen. The validity of this approach is first tested on UO 2 , which oxidizes readily to UO 3 . Under the same reaction conditions, PuO 2 is only covered by a chemisorbed layer of oxygen, which desorbs at elevated temperature. The study excludes the stability of any higher binary Pu oxide as a bulk species.