Fine particles of anatase were suspended in solutions of ammonium alum with Al2O3/TiO2 molar ratios from 0. 1: 1 to 7: 1. By spray drying the suspensions and calcining the spray-dried powders, Al2O3 - TiO2 composite particles were obtained. The results show that after the spray drying, coatings of ammomium alum are formed on the surface of the anatase particles, leading to composite precursor powders (CCPs) with larger particle sizes. Upon calcining the CCPs, ammomium alum pyrolyzes to amorphous Al2O3 and anatase transforms into rutile. Both are mainly responsible for the observed particle size reductions as well as the densification of each composite particle. The in-situ formed alpha-Al2O3 and rutile may have higher reactivities, forming aluminum titanate at 1150 degrees C, about 130 degrees C lower than the theoretical temperature for the formation of Al2TiO5 by solid reaction. The reaction between alpha-Al2O3 and rutile starts from the interface between the anatase and the alum coating and mainly takes place in the single particles formed by spray drying. The molar ratio of Al2O3 to TiO2 influences the final crystalline phases in the composite powders, but not stoichiometrically. (c) 2005 Elsevier B.V. All rights reserved.