Preparation of high-surface-area SiO2-ZrO2 by deposition of silicate species on zirconia under hydrothermal conditions in an aqueous ammonia solution was investigated. A fresh precipitate of ZrO(OH)(2) was heated in a pressure vessel with several pieces of quartz glass tube at 100 degreesC. In the measurement of ICP-AES and solid-state Si-29 NMR, it was elucidated that a silica component dissolved from the glass in the basic solution was deposited on the ZrO(OH)(2) precipitate and that the silica content increased with increasing hydrothermal period. The resulting SiO2-ZrO2 had a specific surface area higher than 240 m(2) g(-1) even after calcination at 500 degreesC. In contrast to the pure ZrO2 particles readily aggregating, the silicate species deposited on the surface of ZrO2 particles prevented the agglomeration of the primary particles during calcination, resulting in high-surface-area SiO2-ZrO2.