The performance of copper/alumina in the selective catalytic reduction of NOx by olefins in excess oxygen has been studied by means of FTIR gas phase analysis. Special emphasis was devoted to the formation of harmful byproducts such as N2O, HCN, and NH3. The effect of copper loading, reaction temperature, nitrogen oxides (NO, NO2), hydrocarbons (ethene, propene), and water on activity and on the formation of byproducts has been investigated. Increasing the copper loading from 0.46 to 1.65 wt% CuO resulted in a shift of the maximum activity to lower temperatures and in slightly lower nitrogen yields. NO2 was reduced more efficiently than NO with both reductants, whereas no significant difference in activity was observed when either ethene or propene was used as a reductant. Water addition suppressed catalytic activity and leveled off the influence of copper loading. Substantial amounts of N2O, HCN, and NH3 were observed for copper-containing catalysts, with ethene showing a markedly lower tendency to form HCN and N2O. Addition of water to the feeds eliminated HCN formation and suppressed the production of N2O but had only a marginal effect on NH3 formation. Temperature-programmed surface reaction (TPSR) and in situ FTIR experiments in various atmospheres with catalysts loaded under reaction conditions with dry feeds revealed the presence of surface deposits containing precursor species for ammonia and hydrogen cyanide formation. No such species were found when catalysts were loaded with feeds containing water. Reference TPSR measurements with acetonitrile and ethyl isocyanate in oxygen- and/or water-containing atmospheres showed a temperature dependence of ammonia formation comparable to that observed for the loaded catalyst, giving evidence that nitrile as well as isocyanate intermediates formed on the catalyst surface could be the source of ammonia found in catalytic activity testing. FTIR spectroscopy revealed the presence of nitrogen-containing surface species for catalysts loaded with a dry feed containing NO and propene, while no such species were found when wet feeds were used. Upon heating in H-2/N-2, cyanide species were produced, whereas isocyanate surface intermediates appeared in the spectra after heating in O-2/N-2. The findings are in accordance with a mechanism in which a nitrogen-containing precursor, which can be a nitrile or an oxime species, reacts to surface isocyanate and/or cyanide species. Hydrolysis of these intermediates provides a pathway to NH3.