This paper presents an innovative method called Micro-cell technique to obtain the water retention curves of geomaterials particularly highly swelling clays such as MX-80 bentonite. This method is based on the controlled water content and measured suction using a dew-point potentiometer system. The method allows obtaining the water retention of swelling material with high precision and reproducibility using the same specimen for the whole wetting and drying cycles. The influence of void ratio, temperature, and the hysteresis effects on the retention behaviour of particular clay minerals can be successfully investigated using this technique. The cell has been deisgned to provide the possibility of freeze-drying of the specimen directly inside for performing Mercury Intrusion Porosimetry (MIP). This feature allows the investigation of microstructural characteristics under a given swelling pressure and avoids porosity changes due to stress release. An application of this methodology is presented in order to study the water retention behaviour of MX-80 granular bentonite used in Swiss concept of high-level nuclear waste storage.