Y 2 O 3 : Eu 3+ , Tb 3+ nanopowders were prepared via the sol–gel process, using yttrium pentadionate, europium, terbium nitrates, and ethanol as precursors. After the sol evaporation at 100°C, the formed gel was annealed from 300 to 900°C for 15 min, and the obtained nanopowders were analyzed by X-ray diffraction to characterize the microstructural behavior and the crystalline structure, which was determined as mainly cubic. The crystallite size varies from 8.6 to 30.9 nm, at 500 and 900°C, respectively. Transmission electron microscopy studies were carried out in order to analyze the microstructure and the size of crystallites. Three different Tb 3+ atomic contents (0.005, 0.075, and 0.01 at.%) were used at a fixed Eu 3+ level (2.5 at.%) with the aim of determining the relationship between the co-doping content and the scintillation intensity of the well-known 5 D o → 7 F 2 europium emission band at 611 nm. The results show that at higher Tb 3+ contents, a quenching process occurs between Eu 3+ and Tb 3+ ; however, even with the lowest Tb 3+ content (0.005 at.%), an increment on the scintillation light yield is observed compared with a Eu 3+ monodoped sample.