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Graphene hydrogenation on Ni(111) in the mbar region has been investigated by combining Near Ambient Pressure X-ray Photoemission Spectroscopy and Density Functional Theory calculations. A complete and nearly perfect graphene single layer and an incomplete defective monolayer were exposed to a methanation mixture (H2 + CO and H2 + CO2,respectively) with a large excess of H2. The observed behavior is quite different for the two systems. In the former case, the C 1s spectrum of the initially strongly interacting graphene shows the appearance of new components corresponding to hydrogenated C atoms and their first nearest neighbors. The defective carbon layer, grown in the presence of sulfur contamination and consisting of a mixture of strongly and weakly interacting graphene with a significant amount of dissolved C, initially shows the same C species and then evolves with the formation of sp3carbon and patches of Ni oxide/hydroxide. NiO forms by CO2dissociation, while sp3carbon is indicative of a rumpling of the graphene adlayer induced by hydrogenation. Both species disappear when annealing in H2. Dissociation of H2and graphene hydrogenation is possible thanks to the presence of the Ni substrate which makes the reaction weakly exothermic and significantly reduces the barrier for H2dissociation.