Abstract A possible option for the disposal of high level long lived radioactive waste is to store the waste in galleries excavated at great depth in clays or claystones. In such conditions, the walls of the galleries will be successively subjected to desaturation induced by ventilation followed by resaturation once the galleries are closed. Partial saturation also affects laboratory samples, due to the added effects of coring, transportation, storage and trimming. In this study, a series of tests was conducted to investigate the water retention properties of the Callovo-Oxfordian claystone (in which the ANDRA Underground research laboratory in France has been excavated) under drying/wetting cycles so as to provide a complete description of the changes in water content, volume and degree of saturation from the as-provided state. The results show that water content changes between 3% (at 150 MPa of suction) and 12% at zero suction, with a shrinkage of 1.4% (at 150 MPa of suction) and a 7% swelling at zero suction. The data obtained complete existing data by defining the initial state with respect to the main drying and wetting paths and by providing volume changes along the suction cycles. Hysteresis effects are also evidenced. Irreversible volume changes are assumed to be due to micro-crack generation and incomplete recovery. The changes in degree of saturation with respect to water content appeared to be reversible, an important fact to consider when simulating volume changes in COx.