Phase equilibria between montmorillonite (Swy-2 from Wyoming, USA) and H2O vapor phase were established by using the humidity buffer technique at 0.1 MPa and room temperature (23 ± 0.5°C). The Relative Humidity (RH; equivalent to the activity of H2O) levels of the system were defined between 6.5% and 97.4% by using six different salt-saturated aqueous solutions. There was no direct physical contact between Swy-2 sample and the buffer solution to prevent any metal ion exchanges between them. The reaction time at each humidity buffer was 10 days, after which the mass of the sample was determined and X-ray powder diffraction spectrum was collected. The experiments were carried out in two directions: one is adsorption (i.e., from low to high RH), and the other is desorption (i.e., from high to low RH), and the equilibrium of the system was demonstrated by the adsorption-desorption reversal experiments in terms of both mass change and crystal parameters (d-001 spacing and unit cell volume). When compared our data with those from previous study for Na-saturated montmorillonite, either synthetic or natural, the Na replacement in the crystal structure favored the retention of water, but only under RH higher than about 62%.