On the basis of DRS, ESR, and XPS studies, the nature and the dispersion of nickel species deposited on ceria (by hydrolysis of an aqueous solution of nickel nitrate) have been investigated upon thermal activations in air. Before thermal activation, mainly octahedral Ni-II species belonging to a Ni(OH)(2) deposit are observed by DRS (lambda(1) = 1230 nm). The concomitant presence of some Ni-II ions grafted on ceria (via Ni-II-O-Ce bridges) is suggested (i) in DRS, by a bathochromic shift leading to a lambda(1) peak at 1400 nm, indicating the presence of some Ni-II ions submitted to a weaker crystal field than in unsupported Ni(OH)(2) and (ii) in XPS, by a Ni-II 2p(3/2) peak at 857.4 eV, 0.5 eV higher than the Ni-II 2p(3/2) peak observed for unsupported Ni(OH)(2) according to a noticeable lowering of the Ni-O bond covalence. Some bulk NiO particles are also formed during the sample drying step as revealed by an XPS peak at 854 eV. These NiO particles are ESR silent at room temperature and at -196 degrees C since NiO is an antiferromagnetic oxide with a Neel temperature of 250 degrees C. In agreement with a previous EXAFS study dealing with the Ni/TiO2 system, the calcination in air at 400 degrees C mainly generates small NiO patches dispersed on ceria and giving a XPS Ni 2p(3/2) peak at 855.5 eV. This binding energy is notably greater than that observed with bulk NiO (854 eV), and this shift is due to either a size effect or an electronic interaction with ceria. The Ni-II ions of these small patches are ESR silent, indicating that they are submitted to distorted crystal fields (S = 0). In parallel, the formation of Ni-III defects upon calcination at 400 degrees C is indicated by a base-line drift of reflectance spectra and a XPS peak located at 857.5 eV. The coexistence of NiO particles and of Ni-III-O-x(n-) (x and n 1, 2) complexes grafted on the ceria surface is suggested. A thermal activation in air at 900 degrees C induces the sintering of NiO particles as revealed by a decrease of the Ni 2p(3/2) binding energy (855 eV) and of the normalized surface area ratio of the Ni 2p(3/2) and Ce 3d XPS contributions, which indicates that 60% of the nickel ions are no longer detected on the ceria surface after calcination at 900 degrees C. The agglomeration of adjacent small NiO domains giving a "mosaic-like" NiO material, characterized by an isotropic ESR signal whose position (signal A:g within the range 2.47-2.2) and line width (Delta H-pp = 1200-1750 G) vary as a function of measurement temperature (from room temperature to -196 degrees C) is proposed. In parallel, a well-resolved ESR signal (signal B:g perpendicular to = 2.358 +/- 0.002; g parallel to = 2.018 +/- 0.002; Delta H-pp = 10-15 G) indicates the presence of bulk Ni-III ions, magnetically diluted inside ceria. These bulk Ni-III ions could be generated either during the sintering of adjacent ceria particles or via bulk diffusion mechanisms.