CeO 2 thin films were deposited on quartz substrates by using the rf-sputtering technique. The 80-keV Ni - ion-implanted and, subsequently, post-annealed films have shown the formation of Ni oxide and Ni metallic phases at 7at% of Ni concentration. Such secondary phases were dissolved by swift heavy ion irradiation with 200-MeV Ag +15 ion beams. Structural properties, surface roughness, and magnetic behavior of the samples were investigated by X-ray diffraction, atomic force microscopy, and hysteresis loop measurements, respectively. Dissolution of secondary phases has been discussed in the light of irradiation-induced local temperature rise and energy loss processes.