Si/mullite/BSAS EBC were plasma sprayed on SiC/SiC and their oxidation and corrosion behavior were investigated in dry oxygen and water vapor at 1200 °C. A modified thermal-oxidation kinetic model was used to estimate the permeability of water vapor and dry oxygen in the mullite/BSAS and SiO2 layer, respectively. The contribution of EBC to the oxidation/corrosion process and that of SiC/SiC substrates were extracted. The method of permeability calculation and contribution extraction in this work provides a direct evaluation of the protective effect of EBCs and can be used as a criterion to compare different EBC systems.