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Several reactions which take place in clay minerals at elevated temperatures were investigated by D.T.A. and continuous high-temperature X-ray diffraction. The combination of these techniques was used to examine the energy changes and phase transformations that occur in kaolinite, halloysite, illite, and montmorillonite at temperatures up to 1450°C. Diffraction data obtained from these minerals as they were heated are used to interpret the nature of the structural transitions, and new phase developments in two-layer type clay minerals are compared with similar developments in three-layer type clays. Continuous diffraction is used to detect changes that take place in the firing history of a clay in which small amounts of chemical impurities or other mineralizing substances are present or have been added. Several chemical impurities were added to each of the clays to determine their ability to either enhance or retard structural transition and new mineral development upon firing. The effects which these additives have on predicted structural changes are illustrated and are interpreted in lieu of chemical and structural requirements.