CO oxidation 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 using a stoichiometric CO-O(2) mixture. The activity of Pd supported on alumina catalyst was improved by increasing barium loading, but the negative effect was observed for Pd on ceria-zirconia mixed oxide. The role of barium in improving the activity of CO oxidation is considered to weaken the adsorption strength of CO on palladium supported on alumina. On the other hand, barium suppresses the interaction of Pd particles with ceria-zirconia mixed oxide support. (C) 2011 Elsevier B.V. All rights reserved.