Equilibria of oxygen segregation were established using a solid electrolyte cell Ni/ZrO 2(+CaO)/Fe-FeO inside the UHV system. An adsorption isotherm θ = ƒ(p o 2 ) was determined by AES which shows two steps. In a first step the p(2×2) structure is formed in the range 10 −25< p O 2 <10 −21 bar. In the range 10 −21< p O 2 <10 −13 bar the centre sites of that structure are filled. This second step leads to the formation of the c(2×2) structure which is stable in equilibrium with NiO (at p O 2 = 1.6×10 −13 bar) as was confirmed by studying oxygen segregation in the couple Ni/Ni-NiO. An approximate fit of the data with two additive Langmuir isotherms yields the difference of the enthalpy for occupation of the sites of the p(2×2) structure and of its centre sites which is about 40 kJ / mole oxygen.