A detailed structural study of selected compositions in the Bi3Nb1-xZrxO7-x/2 system using combined X-ray and neutron powder diffraction techniques is presented. Two distinct composition regions are evident. A limited pseudo-cubic solid solution range is observed up to ca. x-0.4 and details of the defect structure are presented and compared with conductivity data. At higher Zr content, a polyphasic system is observed, with diffraction patterns dominated by a cubic delta-type phase. Evidence suggests that Zr is excluded from this phase and that the phase separation results in a bismuth rich delta-type phase characterised by a significantly larger unit cell parameter as well as high conductivity. (c) 2007 Elsevier B.V. All rights reserved.