Dehydration is an important process which affects the chemical, physical and mechanical properties of materials. This article describes the thermal dehydration and decomposition of the Sorel cement phase 3Mg(OH)2·MgCl2·8H2O, studied by in situ synchrotron X-ray powder diffraction and thermal analyses. Attention is paid on the determination of the chemical composition and crystal structure of the lower hydrates, identified as the phases 3Mg(OH)2·MgCl2·5.4H2O and 3Mg(OH)2·MgCl2·4.6H2O. The crystal structure of 3Mg(OH)2·MgCl2·4.6H2O is solved and refined by the Rietveld method and a structural model for the 3Mg(OH)2·MgCl2·5.4H2O phase is given. These phases show statistical distribution of water molecules, hydroxide and chloride anions positioned as ligands on the magnesium octahedra. A structural scheme of the temperature induced transformations in the thermal range from 25 to 500 °C is presented.