Ab-initio molecular dynamics (MD) study of oxygen ion diffusion in the oxide ion conducting solid electrolyte delta-Bi3YO6 is presented. Vacancy ordering models were tested by structure relaxation and that for < 111 > vacancy pairs was found to have the highest total energy per atom, while those for < 110 >, < 100 > and random vacancy distributions were found to have similar energies. Evidence for polarization of bismuth charge density is seen for < 100 > and < 110 > vacancy pair configurations, consistent with stereochemical activity of the Bi 6s(2) lone pair. MD studies of oxygen diffusion were carried out at three temperatures. Vacancy pair alignments during these simulations were predominantly in the < 110 > direction, consistent with previous neutron total scattering results. (C) 2013 Elsevier B.V. All rights reserved.