The apparent change in activation energy describing the parabolic rate constant for the passive oxidation of SiC is examined. New data are combined with reevaluated previous results to determine the influences of crystallinity, impurity contamination, and multiple flux mechanisms. The results suggest that the high-temperature transition from interstitial-dominant to network-dominant oxygen transport is a property of amorphous SiO2 scales and does not exist for cristobalite. Highly crystalline scales do not show this transition. Agreement among various studies also suggests that, for high-purity SiO2 scales, there is no difference between the rates of interstitial oxygen transport in amorphous SiO2 and in beta -cristobalite.