The microstructural properties of Au/alpha-Mn2O3 catalysts moderately pretreated under different atmospheres, i.e., O-2, He and H-2, in relation to their activities and stabilities on CO oxidation were investigated. The highest initial activity was obtained for the He-pretreated catalyst, which, however, suffered the most severe deactivation with time on stream. Pretreatment with O-2 resulted in the best stability and had the highest steady-state activity among the three catalysts. TEM results revealed that the pretreatment has a negligible effect on the gold particle size. While the formation of moderate surface oxygen vacancies before reaction was suggested to be responsible for the highest initial activity of He-Au/alpha-Mn2O3, overreduction of the oxide support could explain the inferior activity of the H-2-pretreated catalyst. Based on the XPS and in situ DRIFTS studies, the superior performance of the O-2-pretreated O-2-Au/alpha-Mn2O3 catalyst can be attributed to the creation of a specific oxygen-enriched interface leading to an enhanced metal-support synergy. (C) 2009 Elsevier Inc. All rights reserved.