The catalytic performance of Cu/ZSM-5 and gamma-alumina in the selective catalytic reduction of nitrogen oxides by alkenes in excess oxygen and the formation of potentially harmful by-products such as hydrogen cyanide, cyanic acid, ammonia, nitrous oxide and carbon monoxide have been studied by means of FT-IR-gas phase analysis. Over Cu/ZSM-5 the reduction activity was strongly influenced by the type of hydrocarbon, while there was no significant difference when starting from NO or NO2. In contrast, with gamma-alumina NO2 was reduced more efficiently than NO with both reductants. Water addition strongly suppressed the catalytic activity of gamma-alumina. Regarding the formation of undesired by-products, substantial amounts of carbon monoxide were observed in all experiments, independently of the feed composition. The type of catalyst, the use of either NO or NO2(,) the alkene used as a reductant and water strongly influenced the formation of other by-products. With alumina ethene showed a lower tendency to form HCN as compared to propene and water addition further suppressed by-product formation. This contrasts the findings with Cu/ZSM-5, where HCN production was not significantly altered by the presence of water. On this catalyst HNCO was found additionally for dry feeds, whereas ammonia appeared in the presence of water in the same temperature range. Under special feed gas compositions further by-products, formaldehyde and hydrocarbons, were found over Cu/ZSM-5, whereas none of these compounds were observed over gamma-alumina.