With a fixed n = 10 20 cm -3 the σ of all three systems, with both and n-and ptype doping, shows a metallic-like fall with temperature characteristic of a degenerate semiconductor. For n-and p-doped Bi 2 SO 2 and Bi 2 SeO 2 and n-doped Bi 2 TeO 2 the |S| increase with temperature, whereas the |S| of p-doped Bi 2 TeO 2 shows a shallow maximum around 600 K. Since the n-doped systems have relatively large σ and small |S|, the S 2 term dominates the temperature dependence of the PF and the largest S 2 σ are obtained at higher T . On the other hand, the smaller σ and smaller relative change in S with temperature in the p-doped systems means that the PFs peak at low-to-mid temperatures of < 300 K for Bi 2 SO 2 and approx. 350 and 450 K for Bi 2 SeO 2 and Bi 2 TeO 2 respectively.