The dry reforming of methane to syngas was studied in the temperature range 500-800degreesC on a series of Co/SiO2 catalysts modified by MgO (5-35 wt.%). The materials have been prepared by successive incipient wetness impregnation and characterised by BET, XRD, H-2-TPR, CO2 adsorption and in situ-DRIFT. The formation of a silicate adlayer Mg2SiO4 is observed at high MgO content (30-35 wt.%), which corresponds to a much improved catalytic stability under the severe dry reforming conditions. This phase favours the development of small metallic cobalt particles, preventing their coalescence under reaction conditions. A bi-functional mechanism is proposed which combines the accumulation of oxidizing agents like carbonates and hydrogeno-carbonate adspecies on the catalyst support due to a medium basicity of the layer and the reactivity of small metal particles for methane activation. This concerted process tends to limit coke formation and therefore contribute to the observed catalytic stability. (C) 2004 Elsevier B.V. All rights reserved.