Barium-containing NOx storage catalyst showed serious deactivation under thermal exposure at high temperatures. To elucidate the thermal deterioration of the NOx storage catalyst, four types of model catalyst, Pt/Al2O3, Ba/Al2O3, Pt-Ba/Al2O3, and a physical mixture of Pt/Al2O3 + Ba/Al2O3 were prepared and their physicochemical properties such as BET, NO TPD, TGA/DSC, XRD, and XPS were evaluated while the thermal aging temperature was increased from 550 to 1050 degreesC. The fresh Pt-Ba/Al2O3 showed a sorption capacity of 3.35 wt%/g-cat. but the aged one revealed a reduced capacity of 2.28 wt%/g-cat. corresponding to 68% of the fresh one. It was found that this reduced sorption capacity was directly related to the deterioration of the NOx storage catalyst by thermal aging. The Ba on Ba/Al2O3 and Pt-Ba/Al2O3 catalysts began to interact with alumina to form Ba-Al solid alloy above 600 degreesC and then transformed into stable BaAl2O4 having a spinel structure. However, no phase transition was observed in the Pt/Al2O3 catalyst having no barium component, even after aging at 1050 degreesC.