Two new systems, Bi2O3-Er2O3 and ZrO2-Y2O3(CaO)-Bi2O3 were investigated. The first system has a homogeneous cubic, fluorite phase between 17.5 and 45.5 mol % Er2O3 and can be sintered to densities near 95% at 1200 K. At temperatures between 700 K and 1000 K the highest value of the a.c. oxygen ion conduction in this system is twice as much as the highest value found in the literature. In the second system concentrations of 1–3 mol % Bi2O3 act as an excellent sintering aid for ZrO2-Y2O3 and ZrO2-CaO samples which can be sintered to densities higher than 95% at temperatures of 1350 K. During this procedure a liquid ZrO2-Bi2O3 phase exists from which Bi2O3 partly evaporates with increasing sintering time. The oxygen ion conduction is little affected by the Bi2O3-rich second phase. The influence of annealing procedures up to 1570 K on the conduction in the ZrO2-Y2O3-Bi2O3 system is small despite weight losses up to 4%.