The formation of a porous SiC-depleted region in ZrB2–SiC due to active oxidation at ultrahigh temperatures was characterized. The presence/absence of SiC depletion was determined at a series of temperatures (1300°C–1800°C) and times (5 min–100 h). At T < 1627°C, SiC depletion was not observed. Instead, the formation of a ZrO2 + C/borosilicate oxidation product layer sequence was observed above the ZrB2–SiC base material. At T ≥ 1627°C, SiC was depleted in the ZrB2 matrix below the ZrO2 and borosilicate oxidation products. The SiC depletion was attributed to active oxidation of SiC to form SiO(g). The transition between C formation in ZrO2 (T < 1627°C) and SiC depletion in ZrB2 (T ≥ 1627°C) is attributed to variation in the temperature dependence of thermodynamically favored product assemblage influenced by the local microstructural phase distribution. The growth kinetics of the SiC depletion region is consistent with a gas-phase diffusion-controlled process.