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 that is the alternative one. High suctions develop as a consequence of sample retrieval, which affect the hydro-mechanical response of these materials. The water retention properties of both clays, covering a wide suction range and using different complementary techniques, are presented and discussed.