A one-dimensional diffusion problem with prescribed boundary conditions for the oxygen potential at the oxygen(gas)-silica and at the silica-substrate interfaces is employed to obtain the parabolic rate constant for oxidation of Si crystals. The results, using the data for diffusion and solubility of molecular oxygen in silica agree reasonably well with the oxidation kinetics results for Si from Deal and Grove (1965). The measurements for SiC crystals (Costello and Tressler, 1985) lie below these results for Si, even though in both instances, diffusion through the silica overlayer is expected to have been rate controlling. This difference is explained in terms of the lower Si activity at the SiC-SiO2 interface than at the Si-SiO2 interface. The implication of the interface structure is discussed in an attempt to explain the higher activation energy for oxidation of the Si-face (0001), than the C-face (0001) of SiC crystals.