CO-NO-O-2 reaction over Pd supported on barium fixed alumina (Pd/Ba/Al) and ceria-zirconia mixed oxide (Pd/Ba/CZ) catalysts was investigated by XPS, TPR and in situ DRIFT during light-off under a stoichiometric condition. The activity of Pd supported on barium fixed alumina catalysts for CO oxidation as well as NO reduction was improved by increasing the amount of barium loading. In contrast, Ba addition decreased the activity of Pd on ceria-zirconia mixed oxide for CO oxidation, while it had little effect on NO reduction. Enhancement of CO oxidation on Pd/CZ is caused by the activated oxygen supplied from ceria-zirconia mixed oxide to Pd particles. The decrease of the activity for CO oxidation on Pd/Ba/CZ is derived from the reduced supply of the activated oxygen. On the other hand, NO reduction is independent of barium addition for CZ, indicating that the desorption and/or dissociation of NO is not affected by the activated oxygen supplied from CZ under stoichiometric CO-NO-O-2 reaction conditions. (C) 2011 Elsevier B.V. All rights reserved.