La2O3:Eu3+ nanoparticles and hexagonal type-II La2O2CO3:Eu3+ nanospheres were prepared through a hydrothermal reaction followed by a subsequent calcination at 873 K. The shape, size and species of the as-obtained samples can be tuned effectively by controlling the volume ratio of ethylene glycol (EG) to water (W) in solution. When EG > W (by volume), the precipitate was amorphous lanthanum compounds nanorod, which turned into La2O3:Eu3+ nanoparticle after continuously heating. When EG < W (by volume), the precursor was LaCO3OH:Eu3+ and the resulting samples were uniform La2O2CO3:Eu3+ nanospheres. The possible formation mechanism of selective synthesis of the lanthanum nanoparticles was discussed in detail. Furthermore, the gained La2O2CO3:Eu3+ have remarkable stability in the humid air. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.031203jes] All rights reserved.