The high-temperature oxidation of SiG-based structural ceramics is accelerated in the presence of KCl vapors. The corrosion products are primarily alkali silicates and the rate of attack is approximately proportional to the vapor pressure of KCl in the atmosphere. When both KCl and H2O are present in the oxidizing atmosphere, the rate of attack is further accelerated. The accelerated attack is related to the more negative free energy change of the fluxing reaction when the Cl can be removed as the more stable HCl molecule. Addition of Cl, to the environment reduces the rate of attack by keeping the alkali bound to the chlorine rather than forming alkali silicates.