In this study, a group of Ba-doped Pd/CeO2 catalysts were prepared by impregnation method and hydrothermally aged at 750 A degrees C for 20 h to investigate the effect of Ba addition on the hydrothermal stability of Pd/CeO2. The hydrothermal stability of Pd/CeO2 was promoted when doped with a moderate amount of Ba. Particularly, the aged catalyst with optimal 1.0 wt% Ba addition showed a significant improvement in CO oxidation activity, even when cofeeding water and carbon dioxide, presenting superior hydrothermal stability. The positive effects of Ba addition on inhibiting the sintering of CeO2 and aggregation of Pd during hydrothermal aging were proposed by combining with the structural properties, DRIFTS and XPS characterizations. More importantly, the Ba addition favored the preserving of oxidized Pd2+ and active oxygens, which played an important role in the enhanced CO oxidation activity of Ba-doped catalysts. [GRAPHICS] .