The chemical vapor deposition of a thin layer of silica on alumina is shown to improve the thermal stability of alumina with respect to sintering and to reduce the loss of surface area of alumina during heating. Benzaldehyde-ammonia titration showed that samples with high stability were almost completely covered by silica. These observations suggest that an ultrathin silica layer, probably a monolayer, thermally stabilizes the surface. Brsnsted acidity was observed even after calcination at 1493 K: it was initially diminished by this calcination, but readily recovered on exposure to a humid atmosphere, whereas conventional solid-acid catalysts are irreversibly deactivated by complete sintering.