To characterise the native defect chemistry of Li 3 OCl, and to quantify the effect of supervalent and subvalent doping on native defect concentrations and on ionic conductivity, we have performed a hybrid density-functionaltheory study of the defect chemistry and aliovalentdoping response of Li 3 OCl that considers all native vacancy, interstitial, and anion antisite defect species within a self-consistent thermodynamic model. In undoped Li 3 OCl, under Li-poor conditions, the dominant negatively defect species is V Li and the dominant positively charged defect species is V Cl , which is qualitatively consistent with previous descriptions of Li 3 OCl as being . Under Lirich conditions, the dominant defect species are O Cl and V Li . The change in highest concentration negativelycharged defect from V Li to O Cl illustrates the potential limitations of considering the defect chemistry of solid electrolytes only in terms of simple Schottky or Frenkelpair defect formation reactions.