Nickel doping has been found to produce weak room-temperature ferromagnetism (FM) in CeO2. The saturation magnetization (Ms) of the chemically synthesized Ce1−xNixO2 samples showed a maximum for x = 0.04, above which the magnetization decreased gradually. For Ce1−xNixO2 samples with x ≥ 0.04, an activation process involving slow annealing of the sample to 500 °C increased the Ms by more than two orders of magnitude. However, no such activation effect was observed in samples with xx for x>0.04, and (ii) the dramatic increase in Ms in the activated Ce1−xNixO2 samples with x ≥ 0.04 and the absence of this behavior in samples with x1−xNixO2 samples with 0.01 ⩽ x ⩽ 0.10 at 5 and 300 K indicates the presence of several paramagnetic species: (i) two magnetically inequivalent Ni2+ ions with the ionic spin S = 1, (ii) one Ce3+ ion with spin S = 1/2, and (iii) three O2− defects with S = 1/2. The spectra of the samples with x2+ EPR line ascribed to dopant ions in substitutional sites, whereas in samples with x ≥ 0.04, there is an additional EPR line attributed to Ni2+ ions occupying interstitial sites. In the activated sample with x = 0.08, the EPR line due to the interstitial Ni2+ ions is completely absent, and only the line due to substitutional Ni2+ ions is present, suggesting that the enhanced FM arises from migration of Ni2+ ions from interstitial to substitutional sites.