Defect clusters in Gd-doped ceria containing up to four oxygen vacancies and eight dopant cations have been simulated by the Mott-Littleton two-region approach. With increasing cluster size, the magnitude of the binding energy of the clusters increases so that the clusters become more and more stable. Moreover, the oxygen vacancies tend to form curved chains in the clusters, and the adjacent oxygen vacancies are preferentially separated by < 110 >/2 in the clusters containing more than three oxygen vacancies. It is suggested that the structure of the nano-sized domains that have been reported in heavily doped ceria can be described as an aggregation and combination of these clusters. (C) 2008 Elsevier B.V. All rights reserved.