The effect of thermal excursion and thermal cycling on the oxidation stability of chemical vapor‐deposited (CVD) and was studied at 1350°C. Thermal cycling alone produced no noticeable change in oxidation kinetics. However, transmission electron microscopy showed that oxide scales grown in cycles consist of alternating layers of and . When the oxidation of CVD or at 1350°C was interrupted with a 1.5‐h annealing in Ar at 1500°C, the kinetics of reoxidation at 1350°C were found to be drastically increased. The and then oxidized essentially at the same rate, which is over 50 times the preannealing rate, and comparable to the expected oxidation rate of these materials at 1500°C. This loss of passivity induced by excursion to higher temperature has also been reported for Si, and is considered an intrinsic instability in materials that oxidize to silica.