A first-principles method is employed to investigate the segregation behaviors of hydrogen and boron in Ni-based and Ni 3 Al-based alloys using two models. Chemical binding energy analysis shows that both boron and hydrogen are able to segregate to the interstices in the Ni phase, Ni 3 Al phase and Ni/Ni 3 Al interface. Boron, however, is bound to its neighbor atoms more tightly than hydrogen in both models and its stable state exists over a broader lattice misfit range compared with hydrogen. The bond order analysis we have proposed reveals the origin of the boron-induced ductility and hydrogen-induced embrittlment at the Ni/Ni 3 Al interface with different lattice misfit. The calculations indicate that hydrogen causes more severe embrittlement at the Ni/Ni 3 Al interface in Ni 3 Al-based than in Ni-based alloys. Furthermore, it is found that the boron-induced ductility and hydrogen-induced embrittlement are changed, and thus controllable, by the lattice misfit. Our results provide a quantitative explanation of many experimental phenomena caused by the addition of boron and hydrogen to Ni-based and Ni 3 Al-based alloys.