The localization and number of the different types of water in two Na-smectites (Laponite and hectorite) were studied as a function of the hydromechanical stresses applied. Water volume variation was obtained by macroscopic oedometric tests. Thermogravimetric analysis (TGA), X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) were used to study water-smectite interactions. The TGA results show that the bulk water content decreases while the adsorbed water content remains practically constant with increasing mechanical stress; hectorite adsorbs less water than Laponite at low hydraulic stress. The proportion of adsorbed water obtained by NMR confirms the TGA data. The interlamellar space and the equivalent water layers decrease with increasing mechanical stress and is always lower in hectorite than in Laponite. Hydromechanical effects on the water-Na-smectite system are in agreement with microtexture changes measured by porosimetry. Differences in the properties of the two clays can be attributed to the higher extension of the layers in hectorite compared with Laponite.