MgO has been found to stabilize Ru against volatilization and agglomeration under oxidizing conditions at temperatures up to 850°C (1560°F). However, before MgO stabilized Ru can be used as an automotive NOx reduction catalyst, it must be put in a form compatible with conditions in the exhaust system. This can be done by impregnating a cordierite honeycomb, which has been washcoated first with high surface area alumina then with MgO, with Ru. Such catalysts were tested under both engine dynamometer and vehicle conditions. Good activity maintenance was found in engine dynamometer tests, but activity was lost rapidly in the vehicle tests. This rapid loss of activity was probably caused by exposure of the catalyst to surface temperatures which exceed the temperature stability limit (∼900°C) of the RuO2-MgO surface bond in current versions of this catalyst. Such temperatures could be encountered on the AMA durability cycle used in the vehicle aging of these catalysts.