Selective hydrogenation of nitroarene compounds is applied in many fields such as agrochemicals, pharmaceuticals, and dyes. Pd-catalyzed hydrogenation of nitrobenzene (PhNO2) and nitrosobenzene (PhNO) could exhibit different selectivities. This was regarded as the evidence to challenge the Haber mechanism for PhNO2 hydrogenation in which PhNO is an important intermediate. In this study, we systematically investigate their hydrogenation mechanisms under realistic reaction conditions based on first-principles calculations. It is found that the weak bonding between the nitro group and the Pd(111) surface leads to the flat-lying chemisorption configuration of PhNO2 and the other intermediates during PhNO2 hydrogenation. In contrast, the strong bonding between the nitroso group and the surface makes PhNO switch its chemisorption mode from flat-lying adsorption under the ultrahigh vacuum condition to vertical adsorption under reaction conditions. For the flat-lying PhNO2, the chemisorbed phenyl group makes hydrogenation easier but hinders N-O bond breaking, resulting in the production of PhNH2 via a direct pathway. Conversely, without the hinderance of the chemisorbed phenyl group, N-O bond breaking and N-N coupling become more favorable during the reduction of vertical PhNO∗ toward the formation of azoxy compound on Pd(111). These results unveil the fact that the difference between the selectivities of PhNO2 and PhNO hydrogenation is independent of the formation of PhNO∗ but dependent on the phenyl group adsorption mode. © 2018 American Chemical Society.