The oxidation resistance of 9% chromium steels, either with 0.5% Si or doped up to 4% Si, was studied at 600 and 950 degreesC under either 1 atm oxygen or Ar-H-2 mixture to elucidate the influence of silicon and oxygen pressure. Particular attention was paid to the initial stages of oxidation, using "in situ" monitoring of oxidation in an X-photoelectron spectroscopy (XPS) apparatus at 600 degreesC and under a low oxygen pressure. In 1 atm oxygen, the oxide film formed on the undoped steel mainly consists of iron oxides, while a chromia film grows on the steels containing 2-4% Si. In Ar-H-2/H2O, the scale on the undoped steel is heterogeneous and made up of both iron and chromium oxides while the pO(2) decrease has no significant influence on the Si-doped steels. During heating of the doped steels, silicon segregates at the outer surface and forms a thin SiO2 film. During further oxidation, the silica film only allows the formation of chromia (with Mn). Under the same conditions, no silicon segregation occurs on the surface of the undoped steel and iron oxides formed when oxygen is introduced. These results clearly demonstrate the beneficial influence of Si on the oxidation resistance of 9% Cr steels. (C) 2003 Elsevier Science B.V. All rights reserved.