A volatility diagram was calculated for temperatures of 1000, 1800, and 2500 K to understand the oxidation of ZrB2. Applying the diagram, it can be seen that exposure of ZrB2 to air produces ZrO2 (cr) and B2O3 (l) over the temperature range considered. The pressure of the predominant vapor species was predicted to increase from similar to 10(-6) Pa at 1000 K, to 344 Pa at 1800 K, and to similar to 10(5) Pa at 2500 K. Predictions were consistent with experimental observations that ZrB2 exhibits passive oxidation below 1200 K, but undergoes active oxidation at higher temperatures due to B2O3 (l) evaporation.