A method of manufacturing a semi-conductor on insulator substrate from an SOI substrate, wherein a Si1-xGex layer is formed on a superficial layer of silicon having a buried electrical insulating layer. A silicon oxide layer is formed on the Si1-xGex layer. The resulting stack of silicon, Si1-xGex and silicon oxide layers is etched up to the buried insulating layer leaving an island of the stack, or up to the superficial layer leaving a zone of silicon and an island of the stack. A mask is formed to protect against oxidation on the etched structure, wherein the protective mask only leaves visible the silicon oxide layer of the island. The germanium of the Si1-xGex layer is condensed on the island to obtain an island comprising a layer that is enriched in germanium, or even a layer of germanium, on the insulating layer, with a silicon oxide layer on top of it.