The interaction of nanocrystalline Ce0.5Er0.5O1.75 mixed oxide with an amorphous Al2O3 in oxidizing and reducing atmosphere up to 1100 degrees C was studied by XRD, TEM, SEM-EDS and BET. Uniform, chemically homogeneous Ce0.5Er0.5O1.75 nanoparticles (2 nm in size) were prepared by microemulsion method and deposited on a high surface gamma-alumina support. The nanoparticles were structurally and chemically stable in the oxidizing atmosphere up to 1100 degrees C, exhibiting only an increase of the mean crystallite size to 9 nm after 3 h treatment. Prolonged heating (24 h) at 1100 degrees C caused partial decomposition of the mixed oxide and reaction of the extracted erbium with the support with formation of hexagonal (P6(3)/mmc) ErAlO3 aluminate. The same hexagonal ErAlO3 occurred also in Er/Al2O3 sample prepared by impregnation of Al2O3 support with an aqueous solution of Er nitrate and subjected to heating in air or hydrogen at 1100 degrees C. In the reducing atmosphere the Ce0.5Er0.5O1.75 reacted with Al2O3 already at 800 degrees C, to form an amorphous surface phase. At 900 degrees C monoclinic (P2(1)/c) (Er,Ce)(4)Al2O9 mixed aluminate was formed with the unit cell volume 4.5% bigger than that of pure Er4Al2O9 phase. After 3 h treatment at 1000 C more than half of the (Er,Ce)(4)Al2O9 aluminate decomposed into two nanocrystalline mixed monoaluminates: tetragonal (I4/mcm) (Ce,Er)AlO3 and hexagonal (P63/mmc) (Er,Ce)AlO3. Nanocrystalline mixed aluminate particles with Er3+ ions placed in well-defined lattice sites and supported at the surface of Al2O3 support, may be interesting as highly efficient active components of optical waveguides amplifiers. (C) 2014 Elsevier B.V. All rights reserved.