Stoichiometric magnesium aluminate spinel, MgAl2O4, contains equimolar proportions of Al2O3 and MgO. Spinel can, however, exhibit significant deviations from this stoichiometric composition. There is considerable disagreement concerning which species compensate for either excess Al2O3 or MgO non-stoichiometry. Here, we use empirical and quantum mechanical (density functional theory) atomistic simulation techniques to investigate the defect chemistry accommodating non-stoichiometry. The incorporation of excess Al2O3 was found to be a lower energy process than the solution of excess MgO. Elevated magnesium and aluminium cation vacancy defect concentrations are predicted in Al2O3 rich spinels, whilst MgO excess is facilitated by a combination of oxygen vacancy and magnesium interstitial defects.