The chemical changes that occurred in a Cu-ZSM-5 catalyst during the selective reduction of NO with i-C4H10 in the presence and absence of O2 Were catalogued. In the presence of excess O2 complete conversion of the NO to N2 and the hydrocarbon to CO2 and H2O occurred and the Cu2+ concentration estimated from the integrated intensity of the electron paramagnetic resonance (EPR) signal was not significantly changed from its initial value. When the oxygen concentration was lowered below the point of stoichiometry, however, both of these conversions decreased modestly, but when O2 was eliminated from the feed both conversions fell precipitously and the acid catalyzed decomposition products of isobutane appeared in the products instead Of CO2 and H2O. These changes were accompanied by corresponding changes in the EPR data. Lowering the O2 below the point of stoichiometry effected a loss of from 30% to 50% of the intensity of the CU2+ signal. Eliminating O2 reduced the signal by several orders of magnitude. Remarkably, these reduced catalysts could be restored to their initial oxidation states by adding excess O2 into the feed stream, even when there was evidence that Cu0 was present. Dealumination accompanied selective reduction even in excess O2, particularly above 623 K. This was probably caused by steaming of the catalyst by the H2O Produced in the reaction.