The effects of thermal aging and H2O treatment on the physicochemical properties of BaO/Al2O3 (the NOx storage component in the lean NOx trap systems) were investigated by means of X-ray diffraction (XRD), BET, TEM/EDX and NO2 TPD. Thermal aging at 1000 degrees C for 10 h converted dispersed BaO/BaCO3 on Al2O3 into low surface area crystalline BaAl2O4. TEM/EDX and XRD analysis showed that H2O treatment at room temperature facilitated a dissolution/reprecipitation process, resulting in the formation of a highly crystalline BaCO3 phase segregated from the Al2O3 support. Crystalline BaCO3 was formed from conversion of both BaAl2O4 and a dispersed BaO/BaCO3 phase, initially present on the Al2O3 support material after calcinations at 1000 and 500 degrees C, respectively. Such a phase change proceeded rapidly for dispersed BaO/BaCO3/Al2O3 samples calcined at relatively low temperatures with large BaCO3 crystallites observed in XRD within 10 min after contacting the sample with water. Significantly, we also find that the change in barium phase occurs even at room temperature in an ambient atmosphere by contact of the sample with moisture in the air, although the rate is relatively slow. These phenomena imply that special care to prevent the water contact must be taken during catalyst synthesis/storage, and during realistic operation of Pt/BaO/Al2O3 NOx trap catalysts since both processes involve potential exposure of the material to CO2 and liquid and/or vapor H2O. Based on the results, a model that describes the behavior of Ba-containing species upon thermal aging and H2O treatment is proposed.