The CO hydrogenation into methane was studied over a Ni/SiO2 catalyst by means of in situ transient techniques (DRIFT, SSITKA) in order to have access to a detailed description of the reacting surface and to understand the ageing process related to the kinetic behaviour at steady state. After an initial period of carbon deposition, fast sintering, and particle smoothing via nickel carbonyl transfer, the reacting surface can be described as a monolayer of nickel carbide, largely covered by CO adspecies with an average bonding stoichiometry of CO/2Ni(s). The rate of hydrogenation is controlled by the probability for a hydrogen molecule to collide with an active site formed of one to two adjacent Ni atoms free from adsorbed CO. The concentration of active sites is statistically determined by the CO coverage. Carbon atoms belonging to the carbidic layer associated to the active site are hydrogenated by hydrogen activated on the free Ni atoms. The regeneration of the carbidic layer is in turn ensured by CO dissociation after methane desorption. A rate equation is presented with physically meaningful kinetic parameters.