Sintering of a Ni/SiO2 catalyst was studied under methanation conditions. The loss of metallic surface, accounting for the catalyst deactivation, was shown to come from the migration of nickel subcarbonyl adspecies formed during the reaction. The catalyst, presenting initially a monomodal Ni particle size distribution, evolved towards a bimodal system consisting of small spherical and large faceted crystals, with a selective development of [111] planes after several hours under reaction conditions. The formation of a nickel carbide monolayer could interfere with the formation of the large particles, possibly by decreasing the concentration of the mobile subcarbonyl adspecies. The best description of the observed sintering process was provided by considering a model with two distinct species, evolving according to the Ostwald-ripening mechanism, and being coupled by mass transport through the mobile nickel subcarbonyl intermediates. Long-term simulations allowed for the prediction of the ultimate state of the catalyst, and solutions aimed at slowing down the loss of nickel surface were proposed.