Singlet and triplet potential energy surfaces for the NiO + H-2 --> Ni + H2O and NiS + H-2 --> Ni + H2S reactions have been investigated by density functional calculations at the B3LYP/6-311 +G(3df,2p)//B3LYP/ 6-31G** level as well as by ab initio CCSD(T), CASSCF, and MRCI calculations for some of the key species. The singlet-triplet intersystem crossing is shown to play a crucial role for both reactions. ne reaction of nickel oxide with molecular hydrogen starts from the formation of a t-ONi-H-2 complex bound by 3.7 kcal/ mol relative to NiO((3)Sigma(-)) and H-2. This is followed by an intersystem crossing (the spin-orbit coupling computed at a representative point CIl of the singlet-triplet intersection is 86 cm(-1)) and the system proceeds via a barrier of 13.3 kcal/mol (transition state s-TS1) in singlet electronic state to form a HNiOH intermediate in singlet or triplet states. t-HNiOH, 9.1 kcal/mol more stable than s-HNiOH, lies 4-4.8 kcal/mol below the reactants. The HNiOH molecule rearranges to a triplet t-Ni-OH2 molecular complex via transition state s-TS2 on the singlet potential energy surface and via singlet-triplet transitions, whereas the spin-orbit coupling for a crossing point CI2 in the transition state vicinity is evaluated as 27 cm(-1). On the last reaction step the complex bound by 3.8 kcal/mol dissociates to Ni(F-3) + H2O without an exit barrier. For the reverse reaction, Ni(F-3) + H2O --> t-HNiOH, the barrier, which occurs at s-TS2 in singlet electronic state, is 12.1 and 4.9 kcal/mol at the B3LYP/6-311 +G(3df,2p) and CCSD(T)/6-311 +G(3df,2p) levels, respectively. The NiS + H-2 reaction begins on the triplet potential energy surface and proceeds via a barrier [transition state t-TS1(S)] of 19.1 kcal/mol relative to NiS((3)Sigma(-)) + H-2 to produce the global minimum-a triplet HNiSH molecule, 18.2 kcal/mol below the reactants. This intermediate dissociates to the triplet Ni atom and H2S via s-TS2(S) and a s-Ni-SH2 complex involving singlet-triplet intersections. The Ni(F-3) + H2S reaction is predicted to rapidly produce the HNiSH molecule, which, in turn, can dissociate to NiS + H-2 overcoming a barrier of similar to 13 kcal/ mol with respect to the reactants. Since the highest barriers along the NiO + H-2 - Ni + H2O and NiS + H-2 - Ni + H2S reaction pathways are similar to 13 and similar to 19 kcal/mol, molecular hydrogen is expected to reduce nickel oxide and NiS to atomic nickel at elevated temperatures.