Abstract: The invention relates to a process for buried localized oxidation of a silicon substrate. The process consists in performing a) a sealing on the surface of the substrate (S), by a first nitriding, by growing a later of silicon nitride forming at least one surface layer, then in performing b) the etching (G1) of a trench (T) intended to receive the buried localized oxidation. A second nitriding is performed c) on the free area of the trench (T) in order to obtain a sealing sc of the walls of the trench (T). An etching (G2) is performed at d) on the bottom wall of the trench (T) by at least partial etching of the silicon nitride layer obtained by second nitriding in order to uncover the substrate material (S). A localized oxidation e) is performed to produce the buried oxidation (OE) of the substrate in the trench. Application to the production of integrated circuits.