Superior functional glasses doped with rare-earth ions have been prepared by controlling the valence states of rare-earth ions. However, recent work has revealed unresolved questions about the controlling mechanism of rare-earth ions valence states. To address these questions, oxide glasses with and without Al2O3 and doped with Eu3+ ions were prepared by a melting process; then, the valence states of Eu3+ ions were investigated during heating under a hydrogen environment. The Eu3+ ions were reduced to Eu2+ only in the glass containing Al3+ ions; the reduction occurred in the center of the glass over a short heating period. It was discovered that the reduction of Eu3+ ions concurrently occurred with the formation of OH bonds which were bound with Al3+ ions. Considering this and the data for the H-2 gas diffusion through the glass, we conclude that diffusing H-2 gas molecules react with Al-O- bonds surrounding Eu3+ ions to form AlOH bonds and reduce Eu3+ ions to Eu2+ via the extracted electrons. When H-2 reacts with a glass structure, that hydrogen has transformed into -OH bonds and the hydrogen concentration in the glass decreases. In order to make up the lost hydrogen, more hydrogen molecules can enter into the glass, resulting in the fast reduction of Eu3+ ions in the center of the glass.