The high-temperature oxidation in air and steam at 923 K was examined with pure iron, Fe-10Cr and Fe-10Cr-0.08C (0~0.03)S steels. The amount of hydrogen dissolved into samples during exposure to steam was measured with thermal desorption spectroscopy (TDS). It was found that the amount of dissolved hydrogen was related closely to the steam oxidation resistance of the steels. In case of pure iron, the thickness of the oxide scale formed in steam at 923 K for 360 ks was comparable to that of the scale formed in air. On the other hand, in case of the Fe-Cr binary alloy and the ternary ferritic steel, the oxide scale was much thicker in steam than in air. It was found that the amount of the dissolved hydrogen was much larger in both the binary alloy and the ternary ferritic steel than in pure iron, and then it leads to the more accelerated oxidation rate in the ternary steels in steam.