The programmed heating of montmorillonite samples is accompanied by their dehydration and dehydroxilation, this being registered by means of Derivatograph. The form of the endothermal effects within the 80–250°C interval can be of simple or double character depending on the nature of exchange cations. Though this phenomenon has been known for a long time, no satisfactory explanation has been provided so far. The herewith paper contains the results of bound water researches at various forms (Na, Ca, Al, Fe) of montmorillonite by means of thermal analysis as well as other physico-chemical methods. It has been found that the form of dehydration endoeffects depends on the state of the active centers of the mineral. In certain conditions, by means of thermal analysis one can determine corresponding quantities of bound water that are in agreement with various kinds of active centers of the mineral sorbent.