Nanoparticles of europium-doped yttrium, oxide (Eu: Y2O3) were synthesized by flame spray pyrolysis. The nanoparticles were separated by centrifugation into two size groups (5-60 nm and 50-200 nm), each characterized by laser induced fluorescence spectroscopy, Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). The fluorescence spectra, the electron diffraction pattern, and the XRD pattern of the large particles were typical of the stable cubic (Mn2O3 type) phase of bulk Y2O3 while those of small particles were quite different and indicated the possible presence of higher density metastable mixed phases-including monoclinic with some indication of a face-centered cubic phase. The size dependence of the particle properties could be attributable to the effect of surface free energy that elevated the internal particle pressure as size decreased. Doping with the lanthanide ion provided a new and useful diagnostic method for determining the crystal structure of flame-synthesized materials. (C) 2005 Elsevier Ltd. All rights reserved.