The selective reduction of NO x with added oxygenates over BaNa/Y and Ag/Y zeolites and Ag/g-Al 2 O 3 takes place via complex reaction pathways with a number of common crucial intermediates. Acetate ions are formed by the oxidation of acetaldehyde over these catalysts. These acetate ions react with NO 2 to form nitromethane which decomposes to HNCO via a dinitro-C 1 intermediate. HNCO hydrolyzes to form NH 3 which can react with HONO to form ammonium nitrite. This NH 4 NO 2 efficiently decomposes to N 2 and H 2 O at 100 8C, and at even lower temperatures in an acidic environment. The neutral surface species are expected to be in equilibrium with their ions. The rate-limiting step in these reaction sequences is the reaction of acetate ions to form nitromethane. When nitromethane is directly added to a NO x stream over Ag/Y, $100% conversion of NO x to N 2 is achieved at temperatures as low as 140 8C. In such schemes, NO acts as a reductant of nitrate ions, ammonium nitrate and nitric acid. The benefits of isotopically labeled compounds in the elucidation of such reaction mechanisms and for providing insights into reaction dynamics are also discussed.