It has been suggested that co-doping ceria with two trivalent ions of different sizes to minimize lattice strain produces materials with better ionic conductivity. To investigate the thermodynamic basis of such behavior, enthalpies of formation at room temperature of samarium-doped ceria (Ce(1-x)Sm(x)O(2-0.5)x with 00.05 for singly doped ceria systems can be attributed to defect associates of trivalent cations coupled with charge-balancing oxygen vacancies. For co-doped Ce1-xNdx/2Smx/2O2-0.5x, there is less destabilization at low x compared to singly doped CeO2 but less stabilization at high x and a shift in the composition of maximum (most endothermic) formation enthalpy toward higher dopant concentration. Enthalpies of defect association of Ce1-xNdx/2Smx/2O2-0.5x are less exothermic than those of singly doped materials. (C) 2012 Elsevier B.V. All rights reserved.