Spectroscopic properties and room-temperature persistent spectral hole burning mechanisms of Eu3+-doped borate glasses were investigated. The depth of the burnt hole increased with the amount of carbon powders (i.e., degree of reducing atmosphere). This was attributed to the formation of defects which can donate free electrons for the photoreduction of Eu3+→Eu2+. Holes survived ≫104 s at 20 K and approximately 40% of hole areas were preserved after annealing at 280 K. The photoreduction of Eu3+ to Eu2+ is a primary mechanism of hole burning and this hypothesis was supported by spectral hole properties, relaxation properties as well as changes in the local structure of Eu3+ in glasses. © 2002 American Institute of Physics.