The reaction, of aluminum borates (9Al(2)O(3).2B(2)O(3) and 2A(2)O(3).B2O3) with aluminum nitride (AIN) have been used as a new chemical route to synthesize alumina-boron nitride (Al2O3-BN) composites. Reaction mechanisms were investigated by TG-DTA and static reaction process. The reactions started at around 1200degreesC and completed at around 1500degreesC. Soaking at temperatures higher than 1800degreesC resulted in the reverse reaction that caused great weight loss. Hot pressing promoted the reactions due to the improved diffusion process. The in situ formed BN phase was in agglomerate shape located at the pockets of Al2O3 matrix particles and this distribution was suggested to be beneficial to the strength of materials with weak phase dispersoids. The fracture surface analysis demonstrated that the main fracture mode was transgranular, indicating the existence of a strong Al2O3 network in the in situ synthesized composites. The prepared composites exhibited high strength, low Young's modulus and high strain tolerance. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.