Abstract Atomistic simulation calculations are used to predict the lowest energy defect cluster geometries for M 3+ dopant cations adjacent to a cobalt vacancy in CoO. Both simple dimers (one dopant ion and one vacancy) and trimers (two dopant ions and one vacancy) are considered. The results suggest that small radius dopant ions prefer a cluster geometry in which they are in a second cation neighbour position with respect to a vacant cobalt site whereas large radius ions prefer a first cation neighbour position. As a consequence of this changing geometry preference, the binding energies associated with small and large dopant cations (e.g. Al and La) are larger than those associated with intermediate sized ions (e.g. Fe and Sc).