The thermal evolution of a sol-gel derived cobalt aluminate (CoAl2O4) of a spinel-type structure was studied. The samples were prepared by the sol-gel technique using aluminum-sec-butoxide [Al((OBu)-Bu-s)(3)] and cobalt nitrate hexahydrate Co(NO3)(2) x 6H(2)O as starting materials and ethyl-acetoacetate (C6H10O3) as a chelating agent. The powder precursors were annealed at various temperatures ranging from 200 to 900 degrees C. The properties of products were characterized using differential thermal analysis and thermogravimetric analysis (DTA/TGA), UV-vis diffuse reflectance spectroscopy (DRS) and X-ray diffraction (XRD). The UV-vis DRS spectra revealed that in samples annealed at temperatures below 700 degrees C a part of Co existed in a 3 + oxidation state. The accommodation of Co3+ in octahedral cation sites yields with a green color. Thermal treatment at higher temperatures promote the reduction of Co3+ ions to Co2+ and a change of color from green to blue, later being characteristic of Co2+ ions in tetrahedral coordination. The XRD revealed that annealing of the precursor powder at a lower temperature (500 and 600 degrees C) resulted in a crystal spinel-type phase Co2AlO4. Annealing at higher temperatures (700-900 degrees C) induces a reaction of Co2AlO4 with amorphous alumina producing a phase CoAl2O4 having a partially inverse spinel structure, with 6 in the range of 0.198-0.260. The spinel powders obtained at all annealing temperatures were in the nanosize range. (C) 2012 Elsevier B.V. All rights reserved.