The NO reduction by CO over alumina-supported Zu and Cu-Cr has been Investigated by means of transient experiments using step changes in the feed concentration, First, the isothermal reduction by CO and oxidation by NO of the catalysts was studied, Nest, the influence of oxygen on the NO conversion was determined. NO is adsorbed on the oxidized catalysts as nitrite/nitrate complexes and as Cu2+-NO. The nitrite/nitrate complexes decompose only in a reducing atmosphere, whereas NO readily desorbs from Cu2+ in, He, CO is retained on the catalysts in a CO/NO/He atmosphere. It was found that the conversion of NO is not only limited by O-2 in the gas phase, but that NO also competes with subsurface oxygen to oxidize the surface of the catalysts, Ln other words, NO is only converted when the catalysts are in a reduced slate, Since the activity of the Cu-Cr catalyst in NO reduction was found to he higher than that of the Cu catalyst, it is concluded that er helps to keep the Cu in a more reduced state. N2O may be formed under dynamic conditions even when it is not observed in the steady state, It was nicely demonstrated that the step-response experiments allow the calculation of the concentrations of the different surface species. These concentrations strongly depend on the reaction conditions.