Catalytic wet-oxidation of a model domestic wastewater was carried out, and improvement of the performance of the previously developed Mn/Ce and Ru/Ce catalysts was attempted. Combination of both catalysts resulted in an increase in the catalytic activity; Ru/Mn/Ce (1/9) which had an atomic ratio of Mn to Ce of 1/9 and 3 wt % Ru, exhibited the highest activity. The activity of the Ru/Mn/Ce catalysts was independent of their redox property as examined by the decomposition of hydrogen peroxide. The possibility of further improvement of the catalytic activity was discussed.