Adding 1 wt.% of the fluorine-ion conducting (LaF3)(0.95)(SrF2)(0.05) to polycrystalline oxygen-ion conducting (CeO2)(0.80)(LaO1.5)(0.20) solid solution, the effect on the electrical conductivity is studied as a function of sintering temperature. Highly sinterable (CeO2)(0.80)(LaO1.5)(0.20) starting powder was fabricated by co-precipitation using oxalate method and followed by a hydrothermal treatment. The oxide conductivity was measured by DC and AC four-probe techniques at high oxygen partial pressures between 0.21 and I bar. The polycrystalline microstructure of the sintered specimens was investigated by transmission electron microscopy (TEM). The oxide conductivity is slightly increased by the addition of 1 wt.% (LaF3)(0.95)(SrF2)(0.05) to polycrystalline (CeO2)(0.80)(LaO1.5)(0.20), provided the specimens were sintered above 1500 degreesC. At sintering temperatures above 1700 degreesC, (LaF3)(0.95)(SrF2)(0.05) was detected at the grain boundaries of the polycrystalline (CeO2)(0.80)(LaO1.5)(0.20). Obviously, the (LaF3)(0.95)(SrF2)(0.05) generates ion-conducting pathways between the (CeO2)(0.80)(LaO1.5)(0.20) grains causing a higher grain boundary conductivity.