The oxidation behaviour in dry and wet air of H11 steel was studied at 600 and 700°C by Thermo Gravimetric Analysis (TGA) and in situ oxidation in the specimen chamber of an Environmental Scanning Electron Microscope (ESEM) equipped with a hot stage specimen holder. The oxidation kinetics of H11 steels are quite sensitive to the presence of water vapour and, although the final mass gains show a good overall reproducibility, they are locally rather irregular. The morphology and microstructure of oxide scale are complex and often heterogeneous, particularly at 700°C. In situ oxidation tests permit to follow the evolution of oxide scales and to observe several growth modes of oxide scales. The diversity of the observed scale growth modes can explain the complexity and irregularity of scale growth mechanisms. Some additional in situ oxidation tests were performed in wet nitrogen. Morphology and growth modes of oxide scale grown in wet air and wet nitrogen differ strongly.