Comparison of the transport properties to experiments suggest that these calculations can accurately reproduce measurements on single crystals and provide a reference point for consolidated powders and thin films, and that a favourable cancellation of errors results in near-quantitative predictions of the figure or merit. With this in mind, we predict that recent experiments on Bi 2 SeO 2 may have achieved close to the optimum bulk ZT possible, whereas a sixfold improvement in the reported ZT max of Bi 2 TeO 2 , and an industrially-viable ZT > 1 at high temperature, should be possible by optimising the carrier concentration. More interestingly, the calculations predict that, if the heavy p-type doping demonstrated for other oxychalcogenides is possible for Bi 2 SO 2 and Bi 2 SeO 2 , much larger ZT max ∼ 2.5 competitive with SnSe may be achievable.