Oxide ion conductors are researched as potential electrolytes in solid oxide fuel cells (SOFCs). Yttria-stabilized zirconia is currently the most commonly used electrolyte in SOFCs, leading to very high operating temperatures of 800 - 1000 °C, precluding a more widespread use of these devices. In order to reach the goal of lower temperature (< 800 °C) SOFCs new oxide ion conducting materials are sought. Ba3MoNbO8.5 adopts a novel structure, which is a hybrid of the 9R perovskite polytype (A3B2O9) and the palmierite structure (A3B2O8). It is a pure oxide ion conductor under standard conditions with a conductivity of 2.2 × 10-3 S cm-1 at 600 °C.We request 3 days on POLARIS and 1 day on XPDF (I15-1) for neutron and X-ray total scattering experiments on Ba3Mo1-xNb1+xO8.5-x/2 (x = 0, 0.3) materials to elucidate the structure-property relationships in these new solid electrolytes.