AbstractIn studies concerned with the oxida‐tion of metals various gas mixtures are used to establish the chemical potential of non‐metallic components in crystals. CO/CO2 mixtures e. g. are used to estab‐lish oxygen potentials at 1000°C; in the case of manganese it is found that the presence of such gas mixtures does not produce a change of the reaction rate which might be attributed to the dissolution of carbon in MnO. On the other hand sulfidation experiments with iron in H2SH2 at 800°C show that the aux‐iliary gas produces a pronounced in‐crease in the number of point defects ‐a fact in agreement with theoretically derived relations. It is therefore important prior to the interpretation of experimental high temperature corrosion results to establish the extent to which the particular auxiliary gas is soluble in the corrosion products being formed.