The thermal decay of holographic gratings recorded using the conventional two-wave mixing technique has been studied in congruent and nearly stoichiometric LiNbO3 crystal doped with Mn. The activation energies of this process have been determined in the 70-130 degrees C range for congruent and 20-80 degrees C range for nearly stoichiometric crystals, the obtained values being 1.06 +/- 0.03 and 1.10 +/- 0.03 eV, respectively. The kinetics of the OH absorption spectrum has also been studied in undoped nearly stoichiometric LiNbO3 between 40-120 degrees C. The time dependence of the band intensities can be characterized by exponential time constants obeying the Arrhenius-law. The average activation energy, Ea equals 1.1 +/- 0.1 eV is in good agreement with those obtained from the thermal decay indicating that the hologram fixing process in nearly stoichiometric LiNbO3 is governed by proton migration.