There is lingering disagreement over the relative oxidation kinetics of SiC and Si3N4 in the regime of passive oxidation. Various oxidation enthalpies have been reported for these materials, in some cases with suggestions of an increase at higher temperatures. The result is uncertainty over the respective diffusion mechanisms that limit oxidation rates in these materials. It is suspected that this confusion mag be due, in large part, to the effects of impurities from the oxidation environment. Accordingly, this study was aimed at clarifying some of the issues by undertaking the simultaneous oxidation of CVD SiC and Si3N4 within a furnace tube of fused silica known to be very pure. We discuss the results obtained in the interval 1200 to 1500 degrees C, and their implications on the respective rate-limiting mechanisms.