The intragranular segregation of dopant cations in 25 atom % yttrium-doped ceria was explored by energy filtering transmission electron microscopy. Through the comparison between samples sintered at different temperatures, the correlation between the intragranular segregation of dopant cations and the formation of nanosized domains in heavily doped ceria is clearly shown. It is suggested that the segregation can lead to the domain formation, and the oxygen vacancies may also segregate into the domains with the dopant cations. (c) 2006 The Electrochemical Society.