We showed that small Hf NPs etch, rather than immediately passivating when exposed to O 2 , even at T NP far below the melting or boiling temperatures of bulk HfO 2 , demonstrating that processes that remove mass, such as HfO desorption, outweigh any mass gains from oxide layer formation during the initial stages of Hf oxidation. Passivation did eventually occur, which by analogy to silicon, we tentatively attribute to slow buildup of O absorbed in subsurface layers, leading buildup of surface O ads , which in turn triggers HfO 2 nucleation and growth. Several experiments to test this hypothesis were discussed and are underway.