Scanning-Transmission and Transmission Electron Microscopy techniques (STEM and TEM) have been applied to the characterization of nanostructured gadolinium oxides doped with europium synthesized by spray pyrolysis. The High Angle Annular Dark Field (HAADF) - Scanning Transmission Electron Microscopy (STEM) tools have been used to perform a tomographic study to identify morphological characteristics of nanostructured particles, and to differentiate them according to the heat treatments to which these have been subjected. With these techniques it has been possible to confirm the hollowness and porous nature of samples subjected to low temperature annealing (900 degrees C). Moreover, the beginning of the densification and sintering processes in samples subjected to thermal treatment at higher temperature (1100 degrees C) have been evaluated. Chemical analysis by electron energy loss spectroscopy (EELS) and X ray energy dispersive spectroscopy (EDS) carried out in STEM mode have allowed to confirm the high uniformity and the expected chemical composition. The high resolution tools either allowed to confirm the presence of a cubic (Ia3 symmetry) and the monoclinic (c2/m symmetry) phases in the nanostructured particles. (C) 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.