The (0001) and (0001) faces of 6H SiC have been oxidised sequentially at 1100 degrees C and 100 mbar in ultra-dry oxygen of natural isotopic concentration and in ultra-dry oxygen highly enriched in18O. Measurement of the 18O isotopic concentration profiles by nuclear resonance profiling with the narrow resonance at 151 keV in 18O(p, alpha )15N shows that on the carbon-terminated face (0001) the oxidation mechanism is rather similar to that observed on Si(100), but that on the silicon-terminated face (0001) the surface isotopic exchange and oxide formation are superposed. The surface exchange observed during a third dry oxidation, in 16O2, is very similar in magnitude and spatial extent on Si(100), and the two 6H SiC faces, suggesting that at least near the surface the nature and composition of the three oxides are very similar