The ceria-terbia solid solution system has been studied in order to assess whether ultra-small crystallites have any propensity to form stable anion vacancies at their surface. This has been suggested by many authors and has become generally accepted. The presence of anion vacancies should be manifest in lattice expansion brought about by the presence of associated Ce3+ sites. Using X-ray diffraction (XRD), no such lattice expansion was observed here and instead a lattice contraction with reduced crystallite size was observed. Further, X-ray photoelectron spectroscopy (XPS) indicates there is little evidence for the association of any vacancies with cerium cations. (C) 2012 Elsevier B. V. All rights reserved.