The nanoscale Co-Mg-O oxide sample (15 wt% in terms of Co3O4) was prepared from the corresponding xerogel synthesized by the modified sol-gel method. The hydrolysis of asprepared Mg(OCH3)(2) was carried out using the aqueous solution of Co(NO3)(2) precursor. The CoOx nanocrystallites of about 10-20 nm in size were shown to be uniformly distributed within the MgO matrix. The reduction of Co-Mg-O in H-2 flow was found to proceed in two separate stages within the temperature ranges of 200-350 degrees C and 350-600 degrees C. The prepared binary Co-Mg-O system was demonstrated to possess completely reproducible reduction behavior in the consecutive reduction/reoxidation cycles. The phase composition of the sample exposed to both the reducing and oxidative environment was followed by an in situ X-ray diffraction analysis performed at temperatures of 25, 300, 500 and 700 degrees C. The determined lattice parameters for MgO (a = 4.219 angstrom) and Co3O4 (a = 8.110 angstrom) were found to be slightly increased as compared with the values from Powder Diffraction File, so that the formation of joint non-stoichiometric (Mg1-xCox)O and (Co3-xMgx)O-4 phases was suggested. The strong chemical interaction of cobalt oxide with MgO matrix was also evidenced by the data of a diffuse reflectance UV-vis spectroscopy. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.