We present the results of the thermal stability of Mg/Co multilayers in the EUV range. The annealing study is performed up to a temperature of 400°C. The X-ray reflectivity at 0.154 nm is used in order to determine the structural parameters (thickness, roughness and density) of the layers. The measurements of the EUV reflectivity around 25 nm show that the reflectivity decreases when the annealing temperature increases above 300°C. X-ray emission spectroscopy is performed to determine the chemical state of the Mg atoms within the Mg/Co multilayer. The results show a small oxidation after annealing at 305°C, which increases greatly at 400°C. Scanning electron microscopy images of cross sections of the multilayer show a change of the surface morphology above 305°C. This large change of morphology and the oxidation explain the large reflectivity loss.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.