Abstract In this contribution are reported recent results regarding the preparation and the reactivity of different nickel oxide (NiO) monocrystalline surfaces. At first, it concerns a structural investigation of metal formation by hydrogen reduction of (001) NiO. In most cases, Ni nuclei form three-dimensional islands as observed by RHEED and corroborated by AES. At low temperatures, metal stacking, previously described as pure hcp stacking from (112̄0) planes, is shown to develop, in reality, with stacking faults from which fcc stacking is often formed. At high temperatures, metal stacking is retained as within the oxide and, from a comparative transmission electron microscopy investigation, a model for the topotactic nucleus formation is put forward. Secondly, the preparation and the stability of (111) NiO are investigated. Apparent (111) NiO stability is found to be due to the thermal surface segregation of silicon. Even in very low amount, this very mobile impurity segregates at the (111) NiO surface leading to a two-dimensional O-Si-Ni compound.