NO reduces HNO3 to HONO and surface nitrates to nitrites on a BaNa-Y zeolite, and it reacts with NO2 to form N2O3. Data are also presented which show that adsorbed NO+ reacts with water to form HONO. In the presence of NH3 and H2O these processes lead to the formation of ammonium nitrite, which efficiently decomposes near 100 degrees C to N-2 + H2O, effecting the catalytic reduction of NOx to N-2 A criterion for this path is that the optimum yield of N-2 is obtained with an equimolar mixture of NO + NO2. Since ammonium nitrate, which can also form on this catalyst, does not significantly decompose at 200 degrees C, a typical temperature for diesel exhaust, the reduction of nitrate to nitrite serves to regenerate active sites on the BaNa-Y zeolite. (c) 2005 Elsevier Inc. All rights reserved.