The equilibrium oxygen pressures for alloys coexisting with their oxides have been determined at 1000°C using an electrochemical cell with a calcia‐zirconia electrolyte. These pressures were related to compositional determinations of the solid phases and the isotherm at 1000°C. Oxygen pressure over the wustite‐alloy phase field increases from for equilibrium to for the invariant three‐phase field of wustite containing 0.51 a/o (atom per cent) Ni, spinel containing 0.6 a/o Ni, and an alloy containing 79.6 a/o Ni. The oxygen pressure subsequently increased through the spinel‐alloy phase field to , that of the invariant phase field composed of spinel containing a calculated composition of 11.4 a/o Ni, nickel oxide containing 42.5 a/o Ni, and an alloy containing 99.6 a/o Ni. Iron behaved ideally up to the alloy content of 40 a/o Ni, but then exhibited small negative deviations, whereas nickel deviated negatively at alloy compositions greater than 10 a/o Fe. The spinel‐alloy equilibria have been described by a Gibbs‐Duhem calculation and by a structural model for the spinel based on a lattice point defect model.