NiO-MgO samples (NixMgl-xO:x = moles of Ni/(moles of Ni + moles of Mg)) were prepared by impregnation of MgO powder with an aqueous solution of Ni(NO3)(2), followed by calcination at 773 K. The dispersion of Ni ions in these samples has been studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS). XPS of the depth profile of the samples shows that the atomic concentration of Ni and Mg against the depth is almost constant and close to those of bulk concentrations. The analyses by EXAFS and XANES reveal the absence of the preferential segregation of nickel oxide phase or magnesium oxide phase in all samples, suggesting the substitution of Ni ions for Mg ions. A detailed analysis by the curve-fitting method indicates that Ni ions dissolving into MgO are dispersed at random, regardless of the compositions of the samples. These results suggest that these samples form solid solutions over the entire composition range, even by calcination at 773 K.