Understanding C−C bond cleavages has become important in developing a cost-effective technology for the steam reforming of natural gas, the primary form of hydrogen production. Previous studies of C−C bond cleavages in C 2+ steam reforming have focused solely on the C 2+ H x species. Here, we report the density functional theory (DFT) studies of 10 C−C bond cleavages of ethane decomposition (C 2 H x , x = 0−6) on Ir(100). We also investigated the effects of O and OH species that are present in steam ethane reforming (SER) on these cleavages. The DFT results demonstrate that coupling O with the C 2 H x species decreases activation energy, from >1 to 0.3 eV. Therefore, the C−C bond rupture in SER, along the minimum energy pathway, is likely to take place in C 2 H x O. The findings suggest that a good steam reforming catalyst also needs to facilitate the C 2 H x and O coupling, which would not only improve C−C bond cleavage but also prevent C deposition, a major cause of catalyst deactivation. Furthermore, the activation energy and reaction energy surfaces were constructed for 34 C−C bond cleavage reactions, which allow for direct performance comparisons among catalysts and the selection of a catalyst of interest beyond the minimum energy path.