The oxidation behaviour of a Ferritic 10%Cr steel in Ar-H(2)O mixtures was investigated at 650 degrees C. The studies aimed at elucidating the effect of water vapour content as well as the gas flow rate on the mechanisms of oxide scale formation. An important observation of the present investigation is, that H(2) produced by the reaction of water vapour with the steel, can play a significant role in the oxidation process. It affects the possibility to form an external haematite layer and may alter the oxide scale growth rate. The extend by which the H(2) affects the oxidation behaviour depends on the gas flow conditions, the water vapour content and the exposure time. To confirm these observations a number of specimens were oxidized in Ar-H(2)-H(2)O mixtures. This atmosphere guarantees a very low equilibrium oxygen partial pressure, in which H(2) formed by reaction of the gas with the metal, does not substantially alter the thermodynamic properties of the gas.