Partial substitution of lower valent dopant ions (e.g. Al, Ga, Mg, Zn) onto the Si site in the oxide ion conductor La9.33Si6O26 has been reported to increase the conductivity. In this paper, we report combined atomistic simulation and neutron powder diffraction studies of Zn doping with a view to explaining this enhancement. The neutron diffraction studies show that, in agreement with cation size expectations, an enlargement of the unit cell is observed on Zn substitution for Si. In addition, the refinement suggests that a significant number of oxide ions are displaced into interstitial positions at the channel periphery. Atomistic simulation studies show significant local structural changes affecting the oxide ion channels on Zn doping. In particular, a local expansion of the channels is observed, while the tilting of the MO4 tetrahedra is altered by replacing Si with Zn. The presence of significant interstitial oxide ions and both the above local structural changes are consistent with the observed increase in conductivity. (c) 2006 Elsevier B.V. All rights reserved.