The thermal oxidation of metals by water vapour and by oxygen is compared and several ideas are advanced to explain the differences often observed. The incorporation of hydroxyl ions in the oxide is shown to be of importance, but possible only if vacancy or interstitial oxide defects are present. The decomposition rate of H2O is another key-point and is shown to be correlated with the surface acidity of the oxide. Non-acidic p-type oxides could be very passive in water vapour whereas more acidic n-type oxides may oxidize at the same rate in both atmospheres or even more rapidly provided that rapid OH transport can take place within the formed scales.