Themed issue: Synchrotron radiation and neutrons in art and archaeology 2014 PAPER Claire Gervais et al. Degradation mechanisms of reinforcing iron rebars in monuments: the role of multiscale porosity in the formation of corrosion products investigated by X-ray tomography Monuments in stone often contain metallic reinforcements, the stability and degradation state of which determines the entire integrity of the edifice. Understanding their long-term corrosion process is thus an essential step towards a safe and efficient conservation strategy. In this study, we show that combining laboratory and synchrotron tomography at different scales is efficient for getting an overall three-dimensional picture of the iron corrosion products found in iron rebars from Orí eans cathedral. We demonstrate that beside chemical characterization of the corrosion products, the study of the shape of the corrosion products, their spatial distribution within the stone binder and their relationship with the porosity of the binder can bring significant insights into the corrosion process, and particularly help in understanding the multiple roles that porosity plays in corrosion.