The involvement of shales in engineering fields such as the extraction of shale gas or the nuclear waste geological disposal is mainly driven by the low permeability and the high retention properties of these geomaterials. The high capillary forces developed in the matrix of the shales allow the fluid trapping with the consequence that the material remains saturated until significant values of suction. However, in the context of several engineering applications, the shale formations are exposed to relative humidity values which might induce changes in the degree of saturation. As a consequence, the investigation of the air entry value and of the retention behaviour of the material is of primary significance.The following paper presents an investigation on the water retention properties of shales. The experimental technique developed for such analysis is described in detail and selected results are presented. The testing methodology is based on the control of the water content and on the subsequent measurement of the total suction at equilibrium.The volumetric behaviour of the material is monitored in order to compute the retention curve in terms of degree of saturation. The results allow to identify a relationship between the air entry values and the porosity for Swiss shales.