Reduction of NOx to N-2 over zeolite-based catalysts is a multi-step reaction for which transition metal (TM) ions are helpful but not indispensable. The present state of understanding the mechanism is reviewed for the catalyzed NOx reduction with ammonia or alkanes; reduction with acetaldehyde is also sketched, but will be described in detail in a separate paper. A decisive step is the interaction of two reaction intermediates containing N atoms in different oxidation states. A favorable reaction path uses an organic molecule to reduce part of the NOx to ammonia, this product then reacts with NO + NO2 to give N-2. Isotopic labeling shows that each N-2 molecule has one N atom from the NH3 intermediate, the other from NOx. N-2 is formed at room temperature, if an 1:1 mixture of NO and NO2 is in contact with an Fe/MFI catalyst covered with NH3. During NOx reduction with CH4 over Pd/zeolite catalysts, H/D exchange of methane has been observed, indicating that methane is dissociatively adsorbed on Pd-0 clusters that are present in the steady-state of surface oxidation and reduction steps. BaNaY catalysts, containing negligible impurities of TM elements, catalyze NOx reduction with acetaldehyde at 200degreesC. In the prevailing mechanism acetaldehyde is transformed via acetate ions and nitromethane to isocyanic acid, which is hydrolyzed to NH3. Again, N-2 is ultimately produced from NH3 and N2O3 via ammonium nitrite. (C) 2004 Published by Elsevier B.V.