A thermodynamic model was developed to explain the formation of a SiC-depleted layer during ZrB2–SiC oxidation in air at 1500°C. The proposed model suggests that a structure consisting of (1) a silica-rich layer, (2) a Zr-rich oxidized layer, and (3) a SiC-depleted zirconium diboride layer is thermodynamically stable. The SiC-depleted layer developed due to active oxidation of SiC. The oxygen partial pressure in the SiC-depleted layer was calculated to lie between 4.0 × 10−14 and 1.8 × 10−11 Pa. Even though SiC underwent active oxidation, the overall process was consistent with passive oxidation and the formation of a protective surface layer.