Nano-sized domains were studied systematically in 10-30 at.% Gd-doped ceria. By using energy filtering transmission electron microscopy, it was observed that Gd cations could segregate in the domains. Due to the requirement of electrical neutrality, oxygen vacancies may also segregate into the domains with the dopant cations. Through study of electron energy loss spectroscopy, the local ordering of oxygen vacancies in the domains was further confirmed. Furthermore, the nano-sized domains and the local ordering develop with increasing doping concentration. Based on the experimental results, the formation mechanism of the domains and their effect on the ionic conductivity are discussed. (C) 2008 Elsevier B.V. All rights reserved.