ALSTOM Grid is the project owner of the remediation of a former industrial site, located in Saint-Ouen, north of Paris. The industrial activity (power transformer production) started in 1921 and stopped in 2006. The type of pollution is linked with the former activity. It’s an organic pollution: hydrocarbon, PCB and chlorinated volatile organic compounds. The clean-up concerns soil and groundwater. The main specificity of the project is that the remediation is operated inside the existing industrial buildings which must be kept in place and restituted to the owner after the works. The treatment of soil requires excavating soil up to 9 m deep (1 m under the level of the groundwater) inside the buildings. As a consequence, some impressive devices were set up to ensure the stability of the buildings during the clean-up, like support structures of the foundations and strengthening of the building fronts. In the same time, it has to be pointed out that great diversity of clean-up actions is performed on the site: the soil is excavated to be treated on site (bioremediation or chemical treatment) or off site. The treatment of groundwater consists of pumping the oil staying on the surface and oxidizing the dissolved pollution. This project is probably the first experience of this scale in France with multi-contaminated soil and groundwater decontamination in keeping and reinforcing the existing buildings.