Structural and electrical investigations in the Bi3Nb1-xZrxO7-x/2 system have been carried out using X-ray powder diffraction and AC impedance spectroscopy. Several different fluorite phases are observed in the system. At low substitutions (0.0less than or equal toxless than or equal to0.40), a fluorite delta-Bi2O3-type phase is observed with a fully reproducible linear Arrhenius plot up to 800 degreesC. Above this composition range, a second fluorite phase (delta'-phase) is observed, which has a slightly larger unit cell volume than the delta-phase and shows high-temperature conductivity in the same order of magnitude as delta-Bi2O3. The increase in both high- and low-temperature ionic conductivity in the delta'-phase compared to the delta-phase cannot be attributed to the increase in vacancy concentration alone and suggests that the structure of the delta'-phase is better suited to higher O2-ion mobility. (C) 2004 Published by Elsevier B.V.