Ultra-thin oxide layers were grown using ozonated solutions in function of following parameters: immersion time, ozone concentration and pH. The grown layers were etched away and the oxide layer thickness was measured by determining the Si-concentration in the etch solutions by the Molybdenum Blue method. The oxide layer thickness evolves within 10 minutes to a quasi-maximum and depends on the ozone concentration in the range of 1-15 ppm. At a pH of 1.2 (HNO3) the maximum is somewhat higher than at pH = 4.6. An initial oxidation rate (dt(ox)/dt) was determined at an immersion time of 6 seconds and it was found that the initial oxidation rate is linear with [O-3] for both pH-values. This indicates that initially the reaction is surface-controlled.