In-situ metal matrix composites (MMCs) offer significant advantages over conventional MMCs from both a technical and an economic standpoint. In this paper, an in-situ MMC, i.e. Al/(10 vol.% ZrB2+9.2 vol.% Al2O3), is produced starting from Al + ZrO2 + B by reactive sintering and subsequently densified by hot-pressing. The formation mechanism of ZrB2 and Al2O3 in Al matrix is studied by XRD, thermal analysis and microstructural characterization. Reaction kinetics are also investigated based on the results of the reaction mechanism. The properties are evaluated in terms of microstructural characterization, Young's modulus and bending tests. The in-situ processing involves four intermediate steps and the transitional phases are AlB2, Zr(O, B)(2) and (Zr, Al)(B, O)(2) Regarding the reaction kinetics, conversion fraction vs time relationships have been established for the last three intermediate steps. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.