We used a side view transmission electron microscopy (TEM) technique which allows us to study the oxidation process of Si dots on SiO2 systematically for different dot sizes, oxidation times, and temperatures. Using energy filtering TEM (EFTEM) an excellent contrast between Si and SiO2 is achieved, independent from the crystal orientation. Si dots on SiO2 with initial diameters from 10 to 60 nm were oxidized with different oxidation times at 850 degrees C. The resulting shapes of the SiO2 and the embedded Si cores were determined from the EFTEM micrographs. A. strong retardation of the oxidation process compared to planar oxidation as well as a self-limiting effect for long oxidation times are reported. Furthermore a pattern dependent oxidation is observed, depending on the aspect ratios of the dots.