Atomistic simulation calculations based on energy minimisation techniques have been used to study the energetics associated with M2O3 solution in CeO2. Results show that the binding energy of an oxygen vacancy to one or two substitutional cations is a strong function of dopant cation radius; small dopant ions prefer to occupy first neighbour sites, large dopant ions prefer second neighbour sites. The crossover occurs at approximately Gd3+. which also exhibits the smallest binding energy. These results are used to predict lattice parameter as a function of defect concentration and comparison is made to experimental values.