SiC is a large band gap semiconductor, promising for high power and high frequency devices. The thermal oxide is SiO2. However, the growth rates of thermal oxide on SiC are substantially slower than on Si, and different along the polar directions (<0001> and <0 0 0 1> in the hexagonal polytypes). Thorough understanding of the oxide growth mechanisms may give us new insights into the nature of the SiO2/SiC interface, crucial for device applications. We have determined growth kinetics for ultra-dry thermal oxidation of 6H SiC at 1100 degrees C for pressures from 3 to 200 mbar. At 3 mbar, the lowest pressure studied, the oxide growth rates along the two polar directions are virtually the same. At higher pressures growth is faster on the carbon-terminated (0001) face. After consecutive oxidations at 1100 degrees C and 100 mbar in 18O2 and 16O2 gases, 18O depth profiles show significant isotopic exchange and oxygen movement within the oxide during oxidation