Microstructure and reducibility of Ce-Er-O mixed oxide deposited on gamma-Al2O3 as preformed Ce0.5Er0.5O1.75 nanoparticles or by co-impregnation using nitrate solution was studied by XRD, TEM, STEM-EDS, BET and TPR-TPO methods. The Ce0.5Er0.5O1.75/Al2O3 system shows high thermal stability in the oxidizing atmosphere, but in hydrogen atmosphere formation of mixed Ce, Er aluminates occurred already at 800 degrees C. The system exhibits strongly enhanced low temperature reducibility (below 650 degrees C) in relation to CeO2/Al2O3, but reduction at high temperatures >800 degrees C, with formation of aluminates, caused partial "deactivation" of the system by fixing a part of Ce ions at 3+ oxidation state, not susceptible to reoxidation at 550 degrees C. Oxidation at 1000 degrees C is required to decompose aluminate phases and reoxidize most Ce3+ back to Ce4+ state, but then drastic loss of the low temperature reducibility is noticed. The nanocrystalline Ce-Er-O/Al2O3 system prepared by the impregnation of the alumina with the aqueous solution of nitrates appeared to be chemically inhomogeneous and less stable in both oxidizing and reducing atmospheres. Moreover, it shows much worse low temperature reducibility than Ce0.5Er0.5O1.75/Al2O3. 2015 Elsevier B.V. All rights reserved.