The swelling property of smectite is dominated by the hydration of exchangeable cations in the interlayer spacing (‘interlayer hydration’). By investigating systematically the swelling behavior of various exchangeable cations with different valences and ionic radii, the interlayer hydration of smectite was explored. The swelling behavior of Li+-, K+-, Rb+-, Cs+-, Mg2+-, Sr2+-, Ba2+-, and La3+-montmorillonites in undersaturated conditions was measured precisely over the range 50–150°C by in situ X-ray diffraction (XRD) analyses. The systematic swelling behavior of ten homocationic montmorillonites, the aforementioned eight homoionic montmorillonites, plus Na+ and Ca2+ from a previous study, and the cation hydration energies were analysed by studying the changes occurring in the basal spacing and the 001 peak width. With decreasing cation hydration energy, swelling curves (i.e. plots of basal spacing vs. relative humidity (RH)) change from continuous (Mg2+, La3+, and Ca2+) to stepwise (Sr2+, Li+, Ba2+, and Na+) to one-layer only (K+, Rb+, and Cs+). For the first two groups, the RH at the midpoint between the one- and two-layer hydration states increased as the cation hydration energy decreased. Under low RH, with increasing temperature, the basal spacings of Mg-, La-, Ca-, Sr-, Li-, and Ba-montmorillonites decreased continuously to the zero-layer hydration state, whereas Na-, K-, Rb-, and Cs-montmorillonites swelled from the zero-layer hydration state even at the lowest temperature (50°C). A decrease in the basal spacing at the same RH but at different temperatures suggests the existence of metastable states or that the layer-stacking structure changes with temperature. The systematics of the swelling behavior of various homocationic montmorillonites as functions of RH and temperature (<150°C) at 1 atm are reported here.