A series of Ce_1−xEu_xO_2−x/2 mixed oxides was synthesized by coprecipitation. The solids were characterized by means of XRF, S_BET, XRD, UV−vis, and Raman techniques, and their catalytic activities toward CO oxidation were tested. A solid solution, with CeO₂ F-type structure, is formed for europium contents (measured as Eu₂O₃ by XRF) ≤20% wt. For higher contents, the solid solution is not formed, but a physical mixture is detected. The existence of oxygen vacancies in the solids with Eu₂O₃ contents between 3 and 17% wt was demonstrated by the presence of bands at 532 and 1275 cm⁻¹ in their Raman spectra. The catalytic performances of the solids correlate with the amount of these punctual defects in the solid solution.