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The interface reaction between Cu2O and nickel has been investigated and the reaction path has been analysed on the chemical potential diagram for undoped and doped samples. The layer sequence of products Cu2O/Cu/NiO/Ni for the undoped sample and that of Cu2O/Cu/NiO/Cu–Ni alloy/Ni for the doped sample was obtained. The reaction and diffusion path was explained on the chemical potential diagram of the Cu–Ni–O system, based on the assumption that a local equilibrium is attained at the interfaces. The doping effect of NiO in Cu2O was ascribed to a higher mobility of copper in NiO phase due to the higher chemical potential of oxygen at the Cu/NiO interface resulting from the higher chemical potential of nickel in Cu2O. When the reaction time becomes longer, it is expected that the reaction and diffusion path will become similar to that of the doped sample. Because the reaction and diffusion path is a function of time and has a non-equilibrium character, it can be represented and reasonably explained on the chemical potential diagram obtained from a thermodynamic treatment.