Abstract: The natural montmorillonite is modified by the ionic soil stabilizer (ISS) with different concentrations and the isothermal water vapor adsorption tests are conducted for both the raw and modified soils under the relative humidity (P/P₀) ranging from 0.01 to 0.95. The evolution of the dominated factors in the process of hydration of montmorillonite is interpreted by combining the analyses of variation of d₀₀₁ with P/P₀, water retention velocity curves and results of infrared spectroscopy (IR). Finally, the boundary values of P/P₀ in hydration sequences are proposed, and the water retention equations are derived through hydration energy of cations and surface of minerals, respectively. The results show that for the calcium montmorillonite, the exchangeable cations interact with water molecules to form monolayer of hydration shell at the range of 0<P/P₀<0.15~0.2 firstly and then form bilayer at 0.15~0.2<P/P₀<0.45~0.5, followed by hydration on basal surface of crystal layer at 0.45~0.5<P/P₀<0.8~0.9 to form the integrated bilayer water film. The water retention capacity is dominated by the hydratability of interlamellar cations merely at extremely high suction range (ψ>200 MPa), and mainly influenced by the Van der Waals force between basal surface and water molecules when suction is lower (15<ψ<200 MPa). At a certain suction range, ISS weakens the water retention capacity of montmorillonite by changing the specific physic-chemical parameters. The derived water retention equations can accurately predict the test results and also provide a quantitative insight into the mechanism of action by ISS.