Abstract: A method of growing a silicon oxide layer on a silicon substrate by means of a thermal oxidation in a furnace in the presence of a gaseous mixture, said mixture comprising oxygen and Cl2, said Cl2 being generated by an organic chlorine-carbon source, particularly oxalyl chloride. This method is directed to the growth of (ultra) thin silicon oxides and/or the cleaning of a substrate using a low oxidation power. Consequently the method disclosed is especially suited for temperature below 700° C. and for oxidation ambients containing only small amounts of oxygen.