In the context of other post-transition metal oxides. -It would seem that BaSnO 3 leans more towards tolerance than resistance of p-type doping compared to the other posttransition metal oxides: conductivity remains n-type in In 2 O 3 even when doped with Mg to concentrations up to 6.0 × 10 20 cm -3 due to over-compensation by oxygen vacancies; 53 p-type conduction can be achieved in SnO 2 when doped with N (between 1.5 × 10 11 cm -3 to 3.2×10 14 cm -3 hole concentration), although no acceptor defects are predicted to enable performance higher than this; ZnO can reportedly be doped to hole concentrations of 10 18 cm -3 with N or Cu, although mobility is scarcely reported to exceed 1 cm 2 V -1 cm -1 ; 56 while in Ga 2 O 3 , Zn-or Fe-doping simply serves to reduce the n-type carrier concentration rather than inducing p-type carriers, and DFT calculations suggest that p-type doping is infeasible. This modest doping tolerance, combined with the fact that K-doping results in a relatively delocalised hole, rather than the highly localised anion-centred hole polarons that are exhibited in In 2 O 3 , SnO 2 and ZnO, suggest that p-type doping of BaSnO 3 is perhaps worth revisiting in the laboratory. The use of H as a co-dopant for lowering the formation energy of acceptor complexes, as seen in H:Mg-doped GaN, 60 might be a viable route for increasing the concentration of holes in BaSnO 3 and is certainly worth exploring.