Isothermal oxidation behaviour of a hot-work tool steel (X38CrMoV5) was investigated at 600degreesC and 700degreesC in dry and wet air. Growth kinetics were determined by using TGA and oxide scales were characterised by means of SEM (EDS, X-ray mapping) and XRD examinations. Moreover, as the microstructural properties of the studied hot-work steel strongly depend on the carbides precipitates formed during its heat treatment, these carbides were extracted from the X38CrMoV5 matrix and their oxidation behaviour in dry and wet air was also studied. Oxidation behaviour of X38CrMoV5 is very sensitive to the presence of water vapour : a large increase of the scale growth kinetics was observed as soon as the water vapour partial pressure exceeds a value of 9 mbar. Microstructural characterisations showed that scales grown in wet air are porous and sometimes cracked and deformed. They are composed of an external iron-rich oxides scale (hematite alpha-Fe2O3), an internal oxide scale enriched in Cr (spinel oxides (Fe,Cr)(3)O-4) and a narrow zone of internal oxidation. Whereas no significant influence of the water vapour partial pressure has been observed on the oxide scale microstructure (composition, morphology), texture of the superficial hematite scales becomes more pronounced when p(H2O) increases from 9 to 310 mbar. Preferential orientation of alpha-Fe2O3 scales is also favoured by increasing thickness of oxides. On the other hand, the oxidation behaviour of carbide precipitates is rather complex and strongly affected by the presence of water vapour in air.