Performances of Pt and Ph catalysts for the selective reduction of NO were investigated using various reducing agents under high gas hourly space velocity (GHSV). Higher activities were attained when olefins were used for both Pt and Ph catalysts, and when oxygen containing substances such as methyl tert.-butyl ether (MTBE) were used in the case of Rh catalysts. Selectivity toward N-2 for the NO converted was from 7% to 38% for Pt catalysts and from 45% to 67% for Ph catalysts. A comparison of N2O decomposition rates on these catalysts has shown that gaseous N2O is nor a principal intermediate in the pathway for N-2 formation. The high selectivity toward N-2 for Rh/Al2O3 at low GHSV conditions isattributed to an additional catalytic effect of Al2O3, which can reduce NO2 with propylene at temperatures as low as 350 degrees C.