The structure, nonstoichiometry, and electrical conductivity of compositions in the BaBiO3– Ba(Ce1-xGdx)O3-x/2 system have been investigated in an attempt to prepare new mixed (ionic-electronic) conducting oxides. The substitution of Bi into Ba(Ce1-xGdx)O3-x/2 decreases the concentration of oxygen-ion vacancies, and the effective negative charge of the Gd dopant is compensated by the mixed valence of Bi (3+, 5+). For low Bi contents a decrease in ionic conductivity decreases the overall conductivity; however, higher levels of Bi introduce significant electronic conductivity, and for Ba(Bi0.5Ce0.5)O3, σtotal ≈ 1 S/cm at 800 °C in air. Compositions in the Ba(Bi0.5Ce0.5-xGdx)O3 pseudobinary system undergo a B-cation order-disorder transformation at 1300–1350 °C for x = 0.5 and at ≈1250 °C for x = 0.4; all other compositions retain a disordered B-site arrangement. While these disordered perovskites exhibit oxygen nonstoichiometry under reducing conditions at elevated temperatures, with the extent of reduction decreasing with increasing Gd content, their ordered counterparts remain close to stoichiometry. The electronic conductivities of this pseudobinary could be fitted to a “band-type” model, and, despite the presence of oxygen vacancies for the lower values of x, no significant ionic conductivity was observed.