We have recently developed a fully ab inito modelling protocol to determine the figure of merit of thermoelectric materials a function of doping level and temperature by combining accurate models for the electrical-transport properties and the lattice thermal conductivity. This procedure has been shown to yield accurate results for a growing number of thermoelectric materials including the chalcogenides SnS/SnSe and GeS/GeSe and the oxide perovskites CaTiO 3 , SrTiO 3 and BaTiO 3 . In this work we apply the same approach to Bi 2 SO 2 , Bi 2 SeO 2 and Bi 2 TeO 2 with both n-and p-type doping to determine the optimal conditions to maximise the ZT . We observe excellent agreement between our calculated properties and recent experimental measurements, and in particular a favourable cancellation of errors between the electrical and thermal transport properties that allows the calculations to yield near-quantitative predictions of the ZT . We find that recent experiments on bulk n-type Bi 2 SeO 2 are likely to have achieved close to optimum performance, whereas the reported ZT of Bi 2 TeO 2 can be improved sixfold by optimising the carrier concentration. Finally, we also predict that heavy p-type doping of Bi 2 SO 2 and Bi 2 SeO 2 could produce much larger ZT ≈ 2.5 competitive with the benchmark thermoelectric SnSe.