Values of the sorption energies of single molecules of water on all available sorption sites of amorphous cellulose I have been obtained by conformational analysis. The sorption energies are equated to the total energy (Etot) of interaction between the water molecule and all the atomic groups of the cellulose. Van der Waal, H-bond and electrostatic energies comprised the Etot. The interference of water molecules on two vicinal sorption sites were also obtained and sites in which such interference can occur were identified for amorphous cellulose. Curves relating Etot to percentage equilibrium moisture content are reported as well as exact sorption isotherms constructed as Etot as a function of humidity for the amorphous and crystalline cellulose I. These isotherms were constructed for different relative proportions of amorphous to crystalline cellulose and can be constructed for any of their relative proportions from the isotherms for amorphous-only and crystalline-only cellulose. The sorption energies of the monolayer were all calculated. Curves of energy of sorption of bound water were also obtained by introducing the calculated energy values in equations from already established sorption theories. Sorption capacities of amorphous and crystalline Cellulose I were calculated and the respective isotherms constructed and discussed from the point of view of existing sorption theories.