By using the fluorescence line-narrowing technique, we observed a broad fluorescence band in the vicinity of a resonant line of the 5D0→7F0 transition in an Eu3+-doped SiO2 glass synthesized by a sol-gel process. The comparison with a similar line in an Al2O3–SiO2:Eu3+ sol-gel glass revealed the existence of a chemical cluster of Eu3+ and O2- in the tetrahedral SiO4 network. The broad fluorescence band was attributable to an energy migration among the Eu3+ ions for the site-selectively received excitation energy. Also, based on Yokota–Tanimoto’s energy diffusion model, the fluorescence decay curves for the 5D0→7F2 transition were closely correlated with the energy migration and gel–glass transformation. The gel-shrinkage and reduced interatomic distance between Eu3+ ions due to a thermal treatment at higher temperature definitely resulted in a decrease in the associated lifetime of the initial decay. © 2001 American Institute of Physics.