The partial oxidation of methane to synthesis gas was investigated over the alkaline earth metal oxide supported Co catalysts. Only MgO has proved to be a suitable support. The reaction behavior of the MgO-supported Co catalysts was significantly influenced by the calcination temperature (T,) and Co loading. The 24 wt% Co/MgO catalyst precalcined at 800 degreesC provided a high and stable activity; those precalcined at higher temperatures provided a lower stable activity and those precalcined at lower temperatures a lower stability. Because of the two oxidative states of Co, the structural characteristics of the Co catalysts are strongly affected by the nature of the support, calcination temperature, and Co loading. In the 24 wt% MgO-supported catalysts, only a solid solution of CoO and MgO was identified for T-c > 800 degreesC, while one or two of the following compounds, Co3O4 and MgCo2O4, were additionally detected for T-c < 800 degreesC, Correlations among the species present in the catalyst, its reducibility, and its catalytic performance were established.