Al2O3-SiO2 glasses were prepared for doping with Sm3+ ions using a sol-gel method, followed by X-ray irradiation, in which formation of the Sm2+ ions and persistent spectral hole burning (PSHB) were investigated. The X-ray irradiation brought about hole centers in the oxygen ions bound with Al3+ ions. And the released electrons are captured in the Sm3+ ions to form Sm2+ ions. The X-ray reduced Sm2+ ions are not so stable that they change into Sm3+ by heating at 200 to 300 degreesC. The spectral holes were burned in the F-7(0)-->D-5(0) line of the Sm2+ ions, in which deep holes could be burned within a short laser-irradiation period compared with that of the H-2-gas heated glass. Hole spectra were stable up to room temperature, at which the hole depth was similar to20% of the total fluorescence intensity. Bleaching at the low energy sites of the F-7(0)-->D-5(0) transition was observed during irradiation with the laser. The results definitely reveal that the PSHB is formed by the electron transfer between the Sm2+ ions and the hole centers formed in oxygen ions bound through the excited D-5(0) state.