A report on the thermal oxidation of rough silicon surfaces as measured by spectroscopic immersion ellipsometry and atomic force microscopy (AFM) is given. It was found that, as the thickness of the thermally grown SiO2 overlayer increases, the average radius of the crystalline silicon protrusions (roughness) at the interface decreases. A fractal analysis shows that a simpler surface results from oxidation. The frequency spectra of AFM images are concordant with the fractal analysis and shows that small features oxidize faster. This is in agreement with the predictions of the Kelvin equation that small features are more reactive.