Thermodynamic characteristics of cellulose samples having different crystallinity (X) and type of crystalline allomorph (CI to CIV) have been studied. Using methods of combustion calorimetry and DSC, the accurate values of standard enthalpy, ÄfHo, and free energy of formation, ÄfGo, as well as melting enthalpy of crystallites,ÄHo m,were obtained. To avoid the use of uncertain indexes of crystallinity, a precise thermochemicalmethodwas proposed to determine the actual degree of crystallinity (X) of cellulose samples. It was found that with the decrease of crystallinity degree all linear dependences ÄfHo = f (X) and ÄfGo = f(X) converge at one common point, ÄfHo am = -942.4 kJ/mol and ÄfGo am = - 642.6 kJ/mol, corresponding to amorphous phase cellulose at X = 0. This evidences that the amorphous phase in different cellulose samples has idenfull tical thermodynamic characteristics. On the other hand, the linear extrapolation of these dependences to the values corresponding to X = 1 gives the enthalpy and free energy of formation of different crystalline allomorphs. Based on obtained thermodynamic characteristics, the phase stability of various crystalline forms of cellulose and amorphous cellulose (CA) was estimated; and namely, the relative stability of the allomorphs decreases in the following order: CII > CIVï³ CI > CIII > CA. These results allow to explain the structural phase transitions and reactivity of the cellulose allomorphs under certain conditions.