For the n-type doping typically targeted in experiments, we predict ZT max = 0.33 and 0.45 for Bi 2 SO 2 and Bi 2 SeO 2 both at n = 2.5 × 10 19 cm -3 and T = 900 K. The predicted ZT max of Bi 2 SeO 2 is obtained at a comparable n but a higher temperature to the best experimental ZT max = 0.38 for Bi 1.90 Ta 0.1 SeO 2 . The dependence of the electrical and thermal transport properties on the doping level and temperature is such that even modest ZT values require moderate n > 10 19 cm -3 and high temperatures.