Physico-chemical study of dioctahedral smectites from deposits of various origins carried out by X-ray fluorescent, X-ray diffraction, infrared spectroscopy, thermogravimetric and thermochemical methods. Na-montmorillonites, Ca-montmorillonites, Zn-containing montmorillonite, nontronites, and volkonskoite were identified as a result of complex study of the composition and structure of smectites. Thermochemical study of the processes of dehydration of the studied smectites found that the enthalpy of the removal of adsorbed and interlayer water ranges from 6 to 16 kJ per mole of H2O, the dehydroxylation of
montmorillonites (removal of constitutional water –OH-groups) is significantly more energy-capacious process (166 ± 10 kJ per1 mole of H2O). By high-temperature melt solution calorimetry the standard enthalpies of formation for hydrous and dehydrated hydroxyl-containing smectites were determined. From experimental data obtained for natural samples the enthalpies of formation of the smectite end members with theoretical composition were estimated.