A process for treating a semiconductor-on-insulator structure that has, in succession, a support substrate, a layer of an oxide or oxynitride of a semiconductor material, and a thin semiconductor layer of the semiconductor material. The process includes providing, on the surface of the thin layer, a mask defining exposed regions of the thin layer; providing a layer of nitride or oxynitride of the semiconductor material on the exposed regions of the thin layer; and applying a heat treatment causing at least some of the oxygen in the oxide or oxynitride layer to diffuse through the exposed regions. The nitride or oxynitride layer is provided at a thickness sufficient to provide a ratio of the rate of oxygen diffusion though the exposed regions to that through the regions covered with the mask that is greater than 2.