[en] The oxidation behavior of 9Cr-1Mo ferritic steels has been studied in air, oxygen, and steam at 1 atm pressure at various temperatures. Long-term experiments in air were carried out from 500-800 degree C by measuring the weight gains by interrupting the experiment at regular intervals of time. Short-term experiments in oxygen from 500-950 degree C and in air at 900 and 950 degree C were carried out by continous recording of weight gain versus time in a continuous-recording thermogravimetric balance. Short-term experiments in steam were carried out using a special atmosphere furnace attached to the thermogravimetric balance. In air/oxygen, the weight gains at 700 degree C were lower than those at 600 degree C, while in steam, the weight gains at 800 degree C were lower than those at 700 degree C. This inversion phenomenon was observed for all the three steels viz. 9Cr-1Mo (high Si), 9Cr-1Mo (low Si), and 9Cr-1Mo-Nb steel. Examination of the oxide scales was carried out using SEM/EDAX, AES/ESCA, and X-ray diffraction techniques, and a mechanism is proposed for the occurrence of the inversion phenomenon