Barium zirconate (BaZrO3) is a candidate material for interface engineering of alumina fiber/alumina matrix composites. Al2O3 and BaZrO3 react at high temperatures to form a series of reaction products. The objective of this work was to characterize these reaction products and to investigate their effect on crack propagation. BaZrO3 coating was applied on Al2O3 fibers via a sol-gel route. The characterization was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The reaction products between BaZrO3 coating and Al2O3 (fibers and/or matrix) included ZrO2, barium monoaluminate (BaO·Al2O3) and Ba β-Al2O3 (BaO·7.3Al2O3). In hot-pressed composites, barium aluminate was mainly present in the form of Ba β-Al2O3. The Ba β-Al2O3 is known to have a layer-type structure, which is likely to be propitious for crack deflection. The interfacial reactions are diffusion-controlled solid-state processes, mainly depending on the diffusion of Ba cations. The Ba β-Al2O3 phase can form either through direct reaction between the BaZrO3 coating and Al2O3 or through indirect reaction between a BaO·Al2O3 intermediate phase and Al2O3. The BaZrO3 coating and the reaction products between the coating and fiber/matrix provide multiple weak interfaces, which are likely to result in crack deflection and thus toughness enhancement in an all oxide composite. © 2002 Published by Elsevier Science Inc.