Mass spectrometry was used to study in-situ the role of volatile species in the oxidation of reference materials Cr, Cr2O3, Al, Al2O3, Si, SiO2, Fe and ferritic/martensitic steels (P91 and P92) at high temperatures. All samples were heated to 650°C at 1atm in a mixture of Ar with 10-80% range H2O vapor. Oxidation times varied between 100 and 200h. Cr(g), CrH(g), CrO(g), CrOH(g), CrO 2(g), CrOOH(g) and Cr(OH)2(g) chromium-oxy-hydroxides species were identified during the corrosion of Cr oxidized in a steam atmosphere of Ar+80%H2O for 100h. CrO(g), CrO3(g) and Cr(OH)6(g) species were present in mass loss of a Cr 2O3 sample in similar conditions for 200h of oxidation. However a mass gain was observed when Cr2O2(g) and CrO(OH)4(g) species were present. Simultaneously, thermogravimetric studies of the oxidation kinetics of the samples were made with one in-situ thermobalance. The P91 and P92 steels were studied with afore-mentioned techniques at the beginning of breakaway oxidation. During 100h of oxidation at 650°C in Ar+10%H2O atmosphere: Cr(OH)6(g), CrOOH(g) and CrO2(OH)2(g) chromium oxy-hydroxides volatile species were detected in the P91 steel, and Cr(OH)2(g), Cr(OH) 3(g), Cr(OH)4(g), CrO(OH)2(g) and CrO(OH) 4(g), species in the P92 steel. The morphology/composition and structure of the oxidized steel samples were also characterized using SEM/EDAX and XRD techniques, respectively. © Springer Science+Business Media, LLC 2006.