Belgium investigates the design for disposal of its ‘High-Level Radioactive Waste’ in two deep clay formations; the Boom clay at Mol, considered the reference host formation, and Ypresian clay at Kallo as the alternative one. The water retention properties of these deep low-porosity formations have been investigated for two main reasons. High suctions develop as a consequence of sample retrieval (at depths between 223 and 350 m), which affect the hydro-mechanical response of these materials, especially at low stress levels. In addition, water retention properties have also been studied to better assess possible desaturation effects due to venting of the disposal facility galleries. After a description of their main properties at intact state and their microstructural features (pore size distributions), the water retention properties of both clays covering a wide suction range and using different complementary techniques are presented and discussed. ; Peer Reviewed ; Postprint (published version)