The perovskite-type compound BaPbO3 in the BaP00 system is of considerable interest in materials science and technology owing to its wide-ranging electronic and chemical applications. To determine the thermodynamic stabilities of this compound and related plumbates, the e.m.f. of the galvanic cells using both oxide and fluoride solid electrolytes and with (a) BaPbO3Ba4Pb3O10PbO, (b) Ba4Pb3O10Ba2PbO4 PbO and (c) BaPbO3Pb3O4Pb3O4B aF2 as the test electrodes have been studied over the temperature ranges 756-968 K, 797-1097 K and 695-880 K respectively. The least-squares analysis and further processing of the e.m.f data together with the standard Gibbs energies ΔGf of formation of binary oxides from the literature have yielded the expressions ΔGof(BaPbO3,s)±1.00(kJ mol-1)= -901.56+026596T (K) (1)ΔGof(Ba4Pb3O10,s)±3.5(kJ mol-1)= -3278.95+089356T (K) (2) and ΔGof(Ba2PbO4,s) ±2.08(kJ mol-1)=-1482.10+0.37421T (K) (3)These data together with phase equilibrium studies have been made use of in constructing the BaPbO phase diagram at 800 K. © 1995.