Heat of dissolution of cellulose samples with different crystallinity in cadoxen has been determined by microcelorimetry method. The heat of dissolution has been stated to increase with decrasing crystallinity. The heat of dissolution of cellulose amorphous regions and melting heat of celluloses I and II crystallites have been calculated on the basis of the data obtained. Dissolution heat values for different celluloses occurred to tbe in average 270 kJ/kg. The melting heat of the cellulose II crystallites is approximately 1.2 times higher than that of cellulose I crystallites. This evidences a high enetgetic stability of the cellulose II crystallites, which has an effect of physical properties and reactivity of cellulose.