Europium-doped lanthanum oxide (5 mol % Eu3+:La2O3) was prepared by calcining europium-doped lanthanum hydroxide (5 mol % Eu3+:La(OH)(3)) previously synthesized by a simple hydrothermal method. Interestingly, we observed different emission Eu3+ signatures depending on the phase of the host (lanthanum oxide or hydroxide) by cathodoluminescence. Taking into account that lanthanum oxide easily rehydroxylates in air, for the first time, we report the use of cathodoluminiscence as a novel characterization technique to follow the lanthanum oxide rehydroxylation reaction versus time according to different annealing procedures. Additionally, differential thermal-thermogravimetric analysis, infrared spectroscopy, and X-ray diffraction techniques were used to identify the phases formed from the Eu3+:La(OH)(3) depending on temperature and to study the evolution of La2O3 to La(OH)(3) versus time. The results showed that the higher the temperature and the longer the annealing time, the higher the resistance to rehydroxylation of the Eu3+:La2O3 sample.