Oxygen ion conduction in the Aurivillius phases Bi2Sr2(Nb or Ta)(2)(Ga or Al)O-11.5 has been evaluated in detail. High-temperature. X-ray diffraction studies, coupled with differential scanning calorimetry, electron microscopy, and conductivity measurements were used to clearly demonstrate that Bi2Sr2(NbxTa1-x)(2)(GayAl1-y)O-11.5 with x=0, 1 and y=0, 0.5, and 1 form only multiphase specimens. The origin of the previously observed fast ion conduction is a result of interconnected Bi2O3 grains that undergo a high-temperature phase transformation to the delta-Bi2O3 polymorph. The Aurivillius phase component of the multiphase specimens does not demonstrate any type of order-disorder transformation, however, the phase transformations of the Bi2O3 are clearly observed using high-temperature diffraction and calorimetry. (c) 2005 Elsevier B.V. All rights reserved.