The more firmly held water which is directly co-ordinated to the exchangeable cations in smectites is clearly distinguished by infra-red spectroscopy from the more labile water in outer spheres of co-ordination. Li+, Ca2+ and Mg2+ retain both types of water to higher temperatures than do K+, Na+ and NH4+, and the former group give stable monohydrates in saponite which are probably not completely decomposed till 400–600°C. K-, Li- and Na-saponite rehydrate completely after heating, but NH4-, Mg- and Ca-saponite are irreversibly dehydrated at 600–700°C, by which temperature NH4+ is also decomposed. NH4-, Li- and Mg-montmorillonite fail to rehydrate after heating to 350°C, and changes in their spectra indicate interaction between the exchangeable cations and the lattice. Decomposition of NH4+ gives H-montmorillonite, but the products from Li- and Mg-montmorillonite are also acidic, and give spectra similar to that of H-montmorillonite. Na- and Ca-montmorillonite do not rehydrate after heating to 500–550°C, and K-montmorillonite rehydrates only partially in the range 350–550°C. Dehydration of saponite and montmorillonite at low temperatures causes changes in lattice vibrations which are reversed on rehydration. The spectra of the products of dehydroxylation of montmorillonite and saponite are slightly affected by the exchangeable cation originally present.