We have studied the thermal stability of HfO2 thin layers on germanium and the substrate interface development. HfO2 was deposited on Ge substrates and annealed in O-2 or N-2 at 500 degrees C (substrate temperature). After O-2 anneal, we observed the formation of hafnium germanate, which is stable at 500 degrees C in N-2 as opposed to GeO2 that desorbs as GeO. We believe that this hafnium germanate is an oxygen barrier and as such is at the origin of the much thinner interface between HfO2 and germanium as compared to silicon. In addition, results suggest that the HfGeOx is related to the high interface state density frequently reported for germanium metal oxide semiconductor devices.