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 degrees C-1800 degrees C) and times (5 min-100 h). At T < 1627 degrees 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 degrees 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 degrees C) and SiC depletion in ZrB2 (T >= 1627 degrees 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.