In order to improve thermal stability, an alumina-gallia aerogel was prepared and the catalyst performance for NO reduction with C3H6 was compared with that of an alumina-gallia xerogel. Basically, both were prepared by a sol-gel method with supercritical drying for the former, while with oven drying for the latter. Upon heating at 800, 900, and 1000 degreesC, the aerogel exhibited higher NO conversion than the xerogel at reaction temperature <400C, while NO conversion was lower on the former than on the latter at >500 degreesC. At 450 degreesC, NO conversion was almost the same for these two catalysts. A marked difference was observed upon heating them at 1100 degreesC: the aerogel still maintained quite a high activity, while the xerogel greatly lost it. After heating the aerogel at 1100 degreesC, gamma -phase alumina remained untransformed with its surface area of 80 m(2)/g, while the xerogel was completely transformed to alpha -alumina with its surface area of 6 m(2)/g. The high activity remaining on the aerogel heated at 1100 degreesC was ascribed to its large surface area.