The lifetimes of non-fixed holograms in LiNbO3 crystals with different dopants, doping levels and oxidation states have been measured in the temperature range from 30 to 180degrees C. Two dark decay mechanisms have been identified: proton compensation and electron tunneling. For LiNbO3 crystals with low doping levels, the dominant mechanism of dark decay is proton compensation with the activation energy of 1.0 eV. This mechanism is, independent of dopant, doping level and oxidation state. For LiNbO3 with high doping levels, the prevailing mechanism is electron tunneling with the activation energy much less than that of proton compensation, e.g., 0.28 eV for LiNbO3:0.25 wt% Fe2O3. The dark decay of holograms stored in crystals with medium doping levels is the result of a combination of both effects.