Carbon incorporation on the silicon surface by reactive ion etching slows down subsequent rapid thermal oxidation of the surface. Carbon effects on O-2 partial pressure dependency and activation energy of the oxidation rate were measured. Both the carbon-containing sample and the monitor sample shared the square-root dependency on O-2 pressure and activation energy of about 1.44 eV. The results strongly suggest involvement of oxygen dissociation in the oxidation reaction. The dissociation moderates substantial depletion of [O-2](i) (concentration of molecular oxygen at the silicon/oxide interface) into mild reduction of [O](i) (concentration of atomic oxygen at the interface) which is actually proportional to the oxidation rate. Hence, the slowdown must be attained by severely depleting [O-2](i) at the interface without any further impacts on the basic mechanism of the oxidation. The depletion was brought about by forming an anomalous layer at the silicon/oxide interface during the oxidation. The layer disfavors the presence of oxygen by increasing its free energy, rather than strongly obstructing its transport by heightening the energy barrier against it. The formation of the layer seems to be triggered by carbon incorporation from the substrate into the layer and integration into the oxide network with C-O bonding.