In the reduction of NO by CH4 in the presence of a large excess of oxygen, the conversion to N-2 over a physical mixture of Co/Al2O3 and H-zeolite catalysts was found to be twice as high as that expected if these components acted independently. This synergistic effect was observed with both H-ZSM-5 and H-USY. By comparing the activities of these catalysts for NO oxidation and CH4 reduction of NO2, it was concluded that the synergistic effect could be explained by the participation of Co ions and proton sites in the reaction. NO is oxidized to NO2 over the Co ions, and NO2 is reduced by CH4 to N-2 over the proton sites.