Kinetic studies have shown that the isothermal dehydration of the clay minerals proceeds according to a first order law enabling velocity constants to be evaluated for different temperatures. From these, the Arrhenius parameters have been determined and on this basis, the clay minerals may be classified into three main groups, viz., kaolinites, secondary mica clays and montmorillonites. No significant difference is shown by comparison of the Arrhenius plots for the kaolinites and halloysites but the secondary mica clays decompose more rapidly than the kaolinites at comparable temperatures. Evidence is provided to illustrate that the constants obtained are associated with the basic process involving interaction of hydroxyl groups in the clay mineral lattice. Indirect evidence also suggests that the irreversibility of the dehydration is due to lattice collapse resulting in the formation of highly stable configurations of Si4+, Al3−, and O2− atoms. The first order nature of the dehydration reaction is shown to be the underlying basis of the type of curve obtained in differential thermal analysis. Two important conclusions are that:—(a) The clay mineral is only approximately 70% decomposed at the peak on the thermal analysis curve. (b) Heating rate has a marked effect on the peak temperature. Mathematical equations have been developed to obtain the effect of heating at a constant rate on the progress of a reaction for which the isothermal velocity constants are known. The effect of sample weight and dilution factors are discussed, together with the kinetic problems associated with mixtures of clay minerals.