HCHO species were effectively involved in the steam reforming and the dehydrogenation of methanol over supported copper and Group VIII metal (Ni, Rh, Pd, and Pt) catalysts. It was suggested that the difference in the catalytic performances of copper and Group VIII metals was ascribed to the difference in the reactivity of HCHO species formed in the course of the reactions. On the basis of the mechanisms proposed, novel functions of Pd/ZnO catalysts recently developed were discussed.