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Currently, to characterize the crystallinity of cellulose, such an estimated parameter as the crystallinity index is used, measured by various methods and techniques. The main purpose of this article was to develop a thermochemical method for determining the real degree of crystallinity (X) of cellulose based on the measurement of the enthalpy of wetting. Various cellulose samples, such as MCC, pure cotton cellulose, bleached wood pulps, mercerized celluloses, and viscose rayon fibers, were used. For these samples, the exothermic wetting enthalpy (ΔH_w), the maximum amount of sorbed moisture (A_o), as well as the X-ray index of crystallinity (CrI) were studied. The dependence of ΔH_w on A_o was linear and can be expressed by the equation: ΔH_w = k A_o, where the coefficient k = <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>−</mo></mrow></semantics></math></inline-formula>336 (J/g). After substituting the theoretical value A_o,a = 0.5 (g/g) into this equation, the numerical value of maximum wetting enthalpy ΔH_w,a = <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mo>−</mo></semantics></math></inline-formula>168 (J/g) for completely amorphous cellulose was obtained. As a result, the equation for calculating the real crystallinity degree (X) expressed in mass fractions was derived: X = 1 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mo>−</mo></semantics></math></inline-formula> (ΔH_w/ΔH_w,a). Analysis of the obtained results showed that only the X parameter can characterize the real content of crystallites in cellulose samples, instead of the approximate CrI parameter.