The oxidation of constantan films has been investigated using Cu-Ni( Mn) single films and (Cu-Ni(Mn))-NiCr film configurations by measurements of electrical resistance and Auger electron spectra depth profiling, as well as by thermogravimetric and differential thermoanalytical studies. Oxidation starts under air and argon atmosphere between 300 and 350 degrees C. Under an N-2-H-2 gas mixture a passivating film formed by a double layer of manganese and chromium oxide reduces further oxidation strongly on the Cu-Ni(Mn) film with an NiCr adhesion layer. Further details of the oxidation mechanism are discussed in connection with the film recrystallization. A 60 nm NiCr covering layer protects well against oxidation.