SiC is unique amongst the wide bandgap semiconductors in that the natural thermal oxide is stoichiometric SiO2, as is the case for silicon. The possibility of producing devices such as MOSFET in which thermal SiO2 is used as the gate insulator has motivated substantial work aimed at understanding the morphology and electrical properties of the SiO2/ SiC interface and the processes responsible for thermal oxide growth. The oxide growth kinetics are quite different, parallel and anti- parallel to the crystal polar direction. We review the experimental study of the nature of the thermal oxide grown in ultra- dry oxygen and of the extended interfacial region at the SiO2/ SiC interface on the nominally Si- terminated and C- terminated polar surfaces of hexagonal polytypes of SiC, highlighting how the use of stable isotopic tracing has helped to clarify processes for which kinetics measurements alone do not prove to be sufficiently incisive.