Pt/H-ZSM-5 contains independent sites for the reduction of NOx, (I) acidic sites associated with H-ZSM-5 which selectively reduce NOx to N-2 via stored hydrocarbons at low temperatures and (2) conventional Pt sites which reduce NOx to N-2 and N2O at higher temperatures. Propylene is trapped on acid sites, undergoes polymerization but retains activity for NOx reduction provided both NO and O-2 are present. Neither O-2 or NO will individually react with the adsorbed polymer but together NOx reduction occurs readily supporting the mechanism of enhanced conversion of NOx via NO2. This phenomenon has also been demonstrated with diesel fuel as the added reductant. This understanding suggests a novel strategy for diesel engines in which fuel is pulsed into the exhaust at the conditions where the acidic sites are active (i.e. 185 degrees C) giving enhanced NOx conversion. After aging in the automobile exhaust, deactivation by de-alumination of the acidic sites occurs with a loss of hydrocarbon trapping and low temperature NOx reduction. Thus, improved hydrothermally stable materials are still needed to satisfy the mobile source applications.