The oxidation of a ternary two-phase Cu-30at%Ni-25at%Fe alloy has been studied at 700-900 degreesC in air. The oxidation kinetics are rather irregular, but always slower than those for the oxidation of Cu and Fe at the same temperature. The scales contain an outermost layer of almost pure Cu-oxide plus an inner region composed of a mixture of the oxides of the three components as well as a Ni-Fe spinel. A relatively thin region of internal oxidation of Fe and Ni is also present at all temperatures, below which the alloy remains two-phase. The depth of internal oxidation is significantly smaller than calculated on the basis of the solubility and diffusion coefficient of oxygen in copper, but neglecting the hindering effect by the internal oxide particles. The general scaling of the alloy is similar to what observed for a number of binary two-phase systems. In particular, the occurrence of internal oxidation of Fe and Ni in spite of their high contents is attributed to the restriction of diffusion in the alloy due to the limited solubility of the components.