A small addition of oxygen to hydrogen gas is known to mitigate the hydrogen embrittlement (HE) of steels. As atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen dissociation by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating HE is to inhibit the reactions between molecular hydrogen and the steel surface. To elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen, and oxygen adsorption, dissociation, and coadsorption on the Fe(100) surface were investigated using density functional theory. The results show that traces of O2 would successfully compete with H2 for surface adsorption sites due to the grater attractive force acting on the O2 molecule compared to H2. The H2 dissociation would be hindered on iron surfaces with predissociated oxygen. Prompted by the notable results for H2 + O2, other practical systems were considered, that is, H2 + CO and CH4. Calculations were performed for the CO chemisorption and H2 dissociation on iron surface with predissociated CO, as well as, CH4 surface dissociation. The results indicate that CO inhibition of H2 dissociation proceeds via similar mechanism to O2 induced inhibition, whereas CH4 traces in the H2 gas have no effect on H2 dissociation. © 2014 Wiley Periodicals, Inc.