A volatility diagram of zirconium carbide (ZrC) at 1600, 1930, and 2200 degrees C was calculated in this work. Combining it with the existing volatility diagrams of ZrB2 and SiC, the volatility diagram of a ternary ZrB2-SiC-ZrC (ZSZ) system was constructed in order to interpret the oxidation behavior of ZSZ ceramics. Applying this diagram, the formation of ZrC-corroded and SiC-depleted layers and the oxidation sequence of each component in ZSZ during oxidation and ablation could be well understood. Most of the predictions from the diagrams are consistent with the experimental observations on the oxidation scale of dense ZrB2-SiC-ZrC ceramics/coatings after oxidation at 1600 degrees C or ablation at 1930 and 2200 degrees C. The reasons for the discrepancy are also briefly discussed.