The absence of a chemical reaction at an interface is conventionally thought to be an important criterion in producing a tough ceramic matrix composite (CMC). As a result of this criterion, interphases in CMCs were chosen on the basis of their chemical reactivity. A weak interface results in crack deflection, crack bridging, and, in fiber-reinforced ceramics, fiber pullout, resulting in an increased fracture toughness. In this paper, we present microstructural observations on alumina (Al2O3)-barium zirconate (BaZrO3) laminated composites wherein the reaction products that develop during processing resulted in sharp interfaces and appear to be weak enough to deflect cracks. These in situ reaction products in Al2O3-BaZrO3 laminated composites were characterized with the use of a scanning electron microscope, an electron microprobe, and a transmission electron microscope. The phases that develop, ZrO2, BaO·Al2O3, and BaO·6 Al2O3, produced sharp interfaces and are arranged in a sequence that could be predicted by using information from the phase diagram. © Elsevier Science Inc., 1998.