The reaction of (NO + C3H8 + O-2) can result in selective formation of NO2 over H-ZSM5, Cu,H-ZSM5, Ag,H-ZSM5, and Li,H-ZSM5 catalysts when the concentrations of NO and O-2 are 0.1 and 9%, SV > 60,000 h(-1) (typical for automotive exhausts), and C3H8/NO < 1. Despite stoichiometric excess of reductant hydrocarbon below this limit, the in situ formed NO2 does not react with C3H8, thus conversion of NO to N-2 is negligible. NO can be reduced by C3H8 selectively to N-2 only when C3H8/NO greater than or equal to 1. Contrary to many suggestions the reaction temperature, concentration of oxygen, space velocity, and type of exchange ions have minor influence on the selectivity for N-2. These parameters affect the rates of reactions (NO + O-2), (C3H8 + NOx) and (C3H8 + O-2), therefore they also affect the production of N-2 in the HC-SCR process, but only when the ratio of C3H8/NO permits. The metal-exchanged zeolites were prepared in situ by solid-state ion exchange from H-ZSM5. Despite the low degree of copper exchange (63%), Cu,H-ZSM5 produces substantially more N-2 than H-ZSM5, Ag,H-ZSM5, or Li,H-ZSM5. However, the selectivity for N-2 is lowest over Cu,H-ZSM5, which also produces considerable NO2 in the reaction of (NO + C3H8 + O-2) even at C3H8/NO greater than or equal to 1 Contrary to prior findings, the catalytic activity of Cu,H-ZSM5 for the oxidation of NO by O-2 to NO2 in absence of hydrocarbon was comparable to that of H-ZSM5 at high space velocities (2.3 l g(-1) min(-1)). By replacing 30 and 40% of the protons of H-ZSM5 by Ag+ and Li+ ions in Ag,H-ZSM5 and Li,H-ZSM5, respectively, the catalytic activity for this reaction becomes negligible at temperatures greater than or equal to 100 degrees C. Some mechanistic consequences of these experimental observations are discussed.