We report on the effect of water vapor on the oxidation of ferritic/martensitic and austenitic chromia-forming steels at 600degreesC. The samples are investigated by thermogravimetry, GI-XRD, SEM/EDX, SAM, GDOES, FIB and TEM. The oxidation Fe/Cr and Fe/Cr/Ni steels in H2O/O-2 environment is strongly affected by the vaporization of CrO2(OH)(2). The oxidation behavior at a certain temperature, gas composition and gas velocity is to a large extent determined by the ability of the metallic substrate to supply the oxide with chromium to compensate for the losses by vaporization. The corrosion resistance of chromia-forming steels is enhanced by high Cr concentration, fast diffusion in the steel bulk (ferrite/martensite rather than austenite), and a high density of steel grain boundaries (small grain size). The corrosivity of the environment increases with the concentration of water vapor and oxygen, with the gas velocity and with temperature.