The mechanism of oxidation resistance in SiC‐coated carbon‐carbon composites containing boron‐based inhibitors has been investigated using thermodynamic calculations, thermogravimetric analysis, and electron microscopy. A model is developed based on the formation of a volatile B2O2sub‐oxide in the interior of the composite which condenses to B2O3upon encountering a locally high oxygen partial pressure in coating cracks. The active‐to‐passive transition for the oxidation of elemental boron has been