CoxMg1-xAl2O4 (x = 0-0.8) nanoparticles were synthesized by sol-gel method, and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy and Al-27 solid-state NMR spectroscopy were performed to study the chemical environments of cations in the nanoparticles as a function of cobalt content and annealing temperature. The results show that the crystallite size of the particles is about 20-40 nm. Besides the tetrahedral and octahedral coordinations, the second octahedrally coordinated Al3+ ions are observed in the samples. The inversion parameter (two times the fraction of Al3+ ions in tetrahedral sites) decreases with the increase of annealing temperature and cobalt content. The fraction of octahedral Mg2+ decreases with the increase of Co concentration. The absorption spectra indicate that Co2+ ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The intensity of the absorption peak corresponding to octahedral Co2+ ions (300-500 nm) decreases with increasing annealing temperature. (C) 2012 Elsevier B.V. All rights reserved.