Reduced NiO/MgO, with a NiO content in the range 9.2-28.6 wt%, was found to be a highly effective catalyst for the CO2 reforming of CH4 to CO and H-2 (at 790 degrees C, atmospheric pressure and a space velocity of 60000 cm(3)g-(1)h(-1)). For smaller or higher NiO contents, the yield was smaller, being negligible for 4.9 wt%. In contrast to the other reforming catalysts, the new catalyst has high stability, since in the optimum NiO range the CO yield remained unchanged at 95% for 120 h without any carbon deposition. The formation of a solid solution between NiO and MgO, which was demonstrated by both X-ray diffraction and temperature-programmed reduction, is most likely responsible for the high selectivity and stability in a large range of compositions of NiO/MgO.