Abstract: Dehydration-induced migration of different exchangeable cations toward the layers of nontronite has been studied by Mössbauer spectroscopy. As interlayer water is removed exchangeable cations migrate toward Fe3+ sites in the tetrahedral sheets of the nontronite (IVFe3+) causing them to distort. The amount of distortion is linearly related to the ionic potential (IP) of the exchangeable cations and is greatest for cations with highest IP. Octahedral Fe3+ sites (VIFe3+) are also affected by migration of cations into the pseudohexagonal cavities. As exchangeable cations move into the pseudohexagonal cavities, interaction with VIFe3+ sites increases. The intensity of the outer VIFe3+ Mössbauer doublet increases with respect to the inner VIFe3+ doublet as the IP of the exchangeable cation increases. It appears that the exchangeable cations play a significant role in determining the thermal stability of nontronite.