Abstract: The rehydration and rehydroxylation properties of homoionic rectorites (saturated with Ca2+, Mg2+, Na+, or K+) were further investigated. The rehydration properties of the rectorite were characterized as follows: (1) basal spacings of rehydrated materials after heating above 500°C changed to 22.5 Å for H2O-complexes, 26.85 Å for ethylene glycol-complexes, and 27.65 Å for glycerol-complexes; (2) rehydrated Ca- and Mg-materials exhibited single layer hydrates at <50% RH, and rehydrated K-material showed double layer hydrates at 80% RH; (3) IR absorption spectra due to rehydrated H2O and OH exhibited the same or very close absorption intensities and frequencies to each other; (4) DTA-TGA curves of rehydrated materials indicated that the amount of rehydrated H2O approached about 4.2 wt. %, and about one-half of OH was rehydroxylated after heating at 800°C; and (5) interlayer cations of expandable layer components became non-exchangeable after heating above 500°C. These results suggest the following rehydration mechanism of rectorite: the interlayer cations migrate into the hexagonal holes of the SiO4 network by thermal dehydration. The cations migrated below 400°C easily return to the interlayer space and their original hydrated configurations have been recovered completely on rehydration. However, those migrated above 500°C are fixed to the hexagonal holes but water molecules are regained in the interlayer space. Consequently, electrostatic effects of interlayer cations on formation of water molecule layers are considerably reduced.