The effect of calcination temperature on the CO2 reforming of CH4 over Co/MgO of various Co loadings was investigated and the results can be summarized as follows: (i) the catalysts with Co loadings between 8 and 36 wt.% precalcined at 500 or 800 degreesC provided high and stable activities; (ii) for Co loadings between 4 and 48 wt.%, the high calcination temperature of 900 degreesC yielded low activities or very long activation periods; (iii) a high Co loading (48 wt.%) combined with a calcination temperature of 500 or 800 degreesC resulted in unstable activities. Depending on the calcination temperature and Co loading, one, two or three of the following Go-containing species, Co3O4, MgCo2O4, and (Co, Mg)O (solid solution of CoO and MgO) were identified by combining temperature programmed reduction and X-ray diffraction (XRD) results. Their reducibility decreased in the sequence: Co3O4 > MgCo2O4 > (Co, Mg)O. It is suggested that the high and stable activities observed over most catalysts (except the 48 wt.% one) precalcined at 500 or 8000C as well as the low activities or the long activation periods observed over the 900 degreesC calcined catalysts were induced by the formation of a solid solution between CoO and MgO. The activity decay observed over the 48 wt.% Co catalyst precalcined at 500 or 8000C was most likely caused by the large metallic particles, formed particularly through the reduction of Co-3 O-4 and MgCo2O4. These particles sintered and stimulated coke formation.