Change in the local structure of aluminum cations was investigated during the preparation of silica-alumina mixed-oxide by coprecipitation from a solution of tetraethyl orthosilicate (TEOS) and aluminum nitrate. In the mixture of hydrolyzed TEOS and aluminum nitrate, aluminum cations exist as aqua complex, and promote hydrolysis and condensation of TEOS. When precipitates are formed by adding ammonia, Al-O-Si bonds are formed accompanied with the change in coordination number of Al cations from 6 to 4. At the same time, acid centers, whose number agrees with the number of Al cations, are generated on the surface of the precipitates. A silica-alumina catalyst prepared by coprecipitation with excess ammonia shows high activity in the cracking of cumene. Dispersion of Al in it, is also high, and this would be the reason of the high activity. © 2005 Elsevier B.V. All rights reserved.