The nucleation and growth of alumina layers on two different FeCrAl alloys were compared, taking into account the presence of alloying elements and impurities. Moreover, the oxygen pressure effect on the nucleation, formation and transformation of transition aluminas was considered. Using an "in situ" deflection test, it was possible to determine the kinetics of the gamma --> alpha alumina transformation. It was found that impurities and oxygen pressure play important roles on the alumina morphology, on the growth mechanism and on the oxide film mechanical resistance. The long-term oxidation resistance is also related to morphological and chemical aspects of the substrate/alumina interface.