Well-ordered, Si- and C-terminated beta-SiC (100) surfaces were oxidized at low oxygen pressure and high temperature to elucidate the mechanism of oxidation. The samples were studied by high resolution electron energy loss spectroscopy (HREELS), Auger electron and electron loss spectroscopies, and low energy electron diffraction. Si-terminated surfaces exposed at T (sub)=1050 degrees C to either O-2 or hot filament-activated oxygen (O+O-2*) and then cooled without oxygen were C-terminated and exhibited similar oxidation rates, surface composition, and HREELS spectra. The change from Si- to C-termination slightly shifted the surface optical phonon. Activated oxygen played no discernible role in the removal of the Si overlayer at this temperature. Samples oxidized under similar conditions but cooled under oxygen were Si-terminated, indicating that the surface lost carbon as CO and CO2 during cooldown. The conversion from Si- to C-termination during oxidation did not occur because of self-limiting etching of a Si monolayer, but rather because the oxidation products of silicon volatilized more rapidly than those of carbon. Oxidation at room temperature resulted in formation of a Si oxide layer.