The surface reactivity in NO conversion to N-2 in the presence or absence of ammonia and oxygen of a copper-on-alumina catalyst was studied by pulse and step-change transient catalytic experiments. In the absence of ammonia, chemisorption of NO on the copper sites of the catalyst is significant and the chemisorption is enhanced by the presence of oxygen. However, gaseous O-2 also promotes the formation of small, but not negligible, amounts of N-2 from NO when no reducing agent is present, probably through the formation of an intermediate copper-nitrite complex which further reacts with NO. In the presence of ammonia, chemisorption of NO still occurs but to a lesser extent. It is shown that the initial rate of NO depletion and N-2 formation in NO or O-2 concentration step-change experiments is higher than that observed in stationary conditions. These results indicate that the mechanism of NO conversion over copper-on-alumina in the presence of chemisorbed ammonia involves the preliminary oxidation of NO to a NO2 or nitrite adspecies which then reacts with chemisorbed ammonia. The formation of N2O is observed only when gaseous O-2 is present and it is suggested that N2O derives from a parallel side reaction involving the decomposition of an ammonium nitrate surface intermediate.