The composite nanoparticles and corresponding self-reinforcing composites comprising tricalcium phosphate (TCP) and alumina (Al2O3) were synthesized by simultaneous precipitation from the CaCl2, AlCl3 and (NH4)(2)HPO4 aqueous solutions, using aqueous NH4OH as precipitant. Influences of the precipitating media pH and the Ca/P atomic ratios on phase composition and morphology of the composites were investigated. Results showed that except for the major phases beta-TCP and alpha-Al2O3, there was always a third minor phase in the calcined composites coprecipitated either in neutral or alkaline condition. Formation of beta-TCP is, however, favored at pH 9.2, whereas more of the third phase, mainly AlPO4, is formed under neutral condition. High Ca/P ratios suppress the formation of alpha-Al2O3 phase under alkaline precipitating condition, but the effect is less significant in neutral condition. TEM observation showed that the 'as prepared' composite particles are nano-sized but interconnected to form a network-like morphology. They were changed to a core-shell-like structure after calcination, while their nano-scale dimension was retained. FEGSEM analysis revealed that the alpha-Al2O3 phase in the sintered composite compacts was in the form of fibrils dispersed in the phosphate phases. These in situ formed fibrils impart a unique role in self-reinforcement of the sintered composites. Mechanical measurements showed that the incorporation of alumina reinforced beta-TCP effectively: the flexural strength increased from 15 MPa of the pure beta-TCP to 84 MPa of the composite with 40 wt% of alpha-Al2O3. (C) 2004 Kluwer Academic Publishers.