Experimental evidences are now accumulating and reveal that subsurface species can play an important role in heterogeneous catalysis. Recently. it has been found experimentally that, on the Ni(1 1 1) surface, CH3 is hydrogenated selectively by subsurface hydrogen. We report here a detailed study of this prototype reaction for the heterogeneous catalysis through subsurface sites by using a theoretical approach. The elementary steps involved are identified, and a detailed analysis of the reaction pathways allows us to understand the microscopic mechanism. The results obtained here highlight the general features of the new mechanism of heterogeneous catalysis through subsurface sites.