The structural characteristics and the reduction behavior of the Co/MgO catalysts were investigated using temperature-programmed reduction (TPR) and X-ray diffraction (XRD). The variables investigated included the preparation method and the heat treatment conditions (calcination temperature and time). Depending on these factors, one, two or three of the following Co-containing species, Co3O4, MgCo2O4 and (Co, Mg)O (solid solution of CoO and MgO) were identified. The extent of solid solution formation increased as the calcination temperature and calcination time increased. A much lower calcination temperature was needed to form a solid solution in the impregnated catalysts than in the physically mixed ones. The formation of a solid solution rendered the catalyst less reducible. Finally, the decomposition of CH4, as a probe reaction, was performed and it was found that the amount of carbon deposited decreased with increasing extent of solid solution formation.