Results of measures of X-ray diffraction showed a sequence of chemical and structural changes such as dehydration, decomposition and formation of new phases in clays subjected to gradual increases in temperature. The components of the samples were determined identifying Ca-montmorillonite, vermiculite, muscovite, kaolinite, chlorita, illite, quartz and albite. The structural refinement by the Rietveld method reported the presence of a higher percentage of non-expansive clay compared to the expansive in all samples. At 250°C, Ca-montmorillonite presents a drastic decrease in its interlayer distance, from 1.5 nm to 1.0 nm, due to removal of water from the surface and pores, as well as of water contained in the interlayer region. In the case of vermiculite, this decrease is much smaller (∼0.04 nm), while kaolinite does not present significant decrease in this parameter. Vermiculite structurally collapses to ∼350° C, whereas kaolinite makes it to ∼500° C, while new phases appear, such as orthoclase and anorthite.