Environments containing water vapour are common in many industrial processes, such as power generation systems. Hence, long-term oxidation (1000 h) of P-91 and AISI 430 was studied at 650 and 800°C, in 100% H 2 O atmosphere. The oxidation resistance of the AISI 430 is better than that of the P-91, due to the formation of protective phases on the surface. At 650°C, a scale composed of Fe 3 O 4 , Fe 2 O 3 and (Fe,Cr) 3 O 4 is formed on P-91, although at 800°C the scale is mainly composed of Fe 3 O 4 and (Fe,Cr) 3 O 4 . On the other hand, on AISI 430 the scale is composed mainly of (Fe,Cr) 2 O 3 at 650°C, and at 800°C a layer of Cr 2 O 3 is formed and remains owing to the higher diffusion rate of Cr at this temperature than at 650°C, the latter of which compensates the Cr depletion by the degradation of the chromia scale.