In the reaction-bonded aluminum oxide (RBAO) process, attrition-milled Al/Al2O3 powder compacts are reacted in air to form Al2O3-based ceramics. Typically, ZrO2 powder is added to control the final microstructure and properties, resulting in fine-grained, high-strength materials. In this study, the effects of ZrO2 on the reactivity of RBAO samples were examined using thermogravimetric analysis, X-ray diffractometry, and other techniques. Previously, it was shown that a self-propagating reaction could occur during firing of RBAO in air. This reaction was attributed solely to the fast and highly exothermic oxidation of the aluminum on the surface of the sample, but the results of the current study show that it is related to a reaction between aluminum and ZrO2, forming Al2O3 and Al3Zr, As a consequence, a new reaction sequence for RBAO consistent with these observations has been proposed.