Abstract: The water retention curves of three montmorillonites are measured using the water vapor adsorption method at the high suction range. Two generalized hydration models and corresponding methods to predict the cation exchange capacity (CEC) and specific surface area (SSA) are proposed based on the XRD results and BET theory. Moreover, the microscopic water retention model at extremely high suction range (ψ>250 MPa) is derived from interaction energy between cation and water molecule dipole given that the retention capacity is affected merely by the interlayer cation hydration when RH is below 0.15. The results show that the hydration sequence of montmorillonite depends on the hydration energy of interlayer cation. For the cation with low hydration energy, the water absorbs firstly on external surface of clay tactoids followed by absorption into interlayer. Otherwise, the water absorbs directly within interlayer space. The predicted values of CEC and SSA agree well with the measured ones, and the derived microscopic model, taking into account of the effect of CEC, cation valence and radius on the suction potential, can predict the suction value accurately upon comparison with the reported data in literatures.