ZrB2-Al2O3 in situ composite powders were synthesized at 1550 degrees C via a novel solid-state precursor route using ZrO2, Al, and BN as raw materials. The final ZrB2-Al2O3 composite powders, consisting of large crystal Al2O3 particles (several micrometers) and fine ZrB2 particles (100-400 nm), form an interwoven network and are intimately mixed on a microscopic scale. The formation mechanism for ZrB2-Al2O3 composite powders comprises two steps: aluminothermic reduction of ZrO2, which generates Zr-Al alloy compounds and the coproducts Al2O3, then followed by a series of solid reactions between Zr and BN to give the ZrB, ZrN, and ZrB2 products. The aluminothermic reduction is a fast reaction, while the reaction between Zr and BN to obtain the final ZrB2 product is the limiting step in the conversion process of the precursor powders. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.