Growth kinetics at 1100° are investigated as a function of mole ratio below 10% for (111), (100), (311), and (110) oriented wafers. It is concluded that the considerable increase of the oxidation rate in the presence of is caused by three reasons: The first is enhanced diffusion of O2 and molecules in the oxide. The second is enhanced reactions at the interface, resulting from a catalytic action of . The third is the contribution of to the oxidation, which is formed by the reaction, . The diffusivity of O2 and molecules in the oxide was measured by a technique which is based on dry or wet oxidation after an initial growth. Infrared spectroscopy of the output gas from the furnace and the observation of the interface through a microscope and secondary electron microscope revealed the nature of the enhanced reactions, which are closely related to gaseous etching of Si.