Ferritic/martensitic HCM12A steel specimens were subjected to 190 h of oxidation in Ar + 40% H2O at 650°C and 700°C. Simultaneous measurements of mass spectrometry and thermogravimetric analysis by a mass spectrometer and a thermobalance of symmetrical furnaces were made. Thermogravimetric measurements verified that the mass gain after oxidation at 650°C was 0.07 mg/cm2, whereas at 700°C the mass gain corresponded to ∼1.3 times that at 650°C. Mass spectrometry measurements indicated the presence of only a volatile species of low-intensity chromium (order of ppm) at 700°C (i.e. CrO2(g)) after between 175 and 190 h of oxidation. At reduced oxidation times, no volatile species were observed. We did not observe any gaseous species at 650°C. The morphology, composition, and structure of samples by scanning electron microscopy, backscattered electron detection, X-ray energy-dispersive spectrometry and X-ray diffraction are described.