For undoped Li 3 OCl we predict ionic conductivities of 4.2 × 10 -10 S cm -1 under Li-poor synthesis conditions and 6.7 × 10 -10 S cm -1 under Li-rich synthesis conditions. In both cases the contribution to the ionic conductivity from lithium interstitials is negligible (∼ 10 -14 S cm -1 ). Our calculated ionic conductivities (∼5 × 10 -10 S cm -1 ) are much lower than those previously reported for experimental samples of nominally stoichiometric Li 3 OCl (10 -3 S cm -1 to 10 -6 S cm -1 ) . One possible reason for this discrepancy is that previous experimental data might not have been obtained for phase-pure "stoichiometric" Li 3 OCl. Li 3 OCl is extremely challenging to synthesise, as indicated by the small calculated region of metastability under typical synthesis conditions (fig. ), and it has been suggested that samples reported in the literature as Li 3 OCl may in fact be competing hydrated phases, such as Li 2 OHCl , which are predicted to have higher total conductivities than pristine Li 3 OCl .