The deactivation of Ni/SiO2 catalysts under the conditions of carbon dioxide reforming of methane was studied as a function of different operating parameters. Several techniques have been used : temperature programmed hydrogenation, magnetic measurements, transmission electron microscopy, and thermogravimetric analysis. It was shown that, whatever the pretreatment, nickel carbide constituted the active phase for this reaction, being established in the very initial period of operation. The reaction mixture was found to be able to reduce the nickel phase on stream, The two major aging factors, namely nickel sintering and carbon deposition, were shown to be strongly related to the pretreatment conditions. A model is proposed to explain the influence of the analyzed operating parameters on the nature of the coke formation and on the changes in morphology of the nickel particles. Optimal operating conditions for the above catalyst for CO2 reforming are defined.