Oxide ion conductors have been a focus of intense research due to their application as electrolytes in solid oxide fuel cells. The currently used materials (yttria-stabilised zirconia) require 800-1000°C to operate efficiently. New oxide materials are sought as electrolytes with high conductivity between 450 and 600°C. Ba3NbWO8.5 has high oxide ion conductivity in this range. It was first described as a new hybrid of the 9R perovskite and the palmierite structure. We have grown high-quality crystals and have shown that the true structure is more complex, with extensive disorder in both the cation and anion sublattices. Lab single crystal XRD was not sufficient to characterise fully the oxide ion sublattice, and neutron diffraction is necessary to provide this understanding, essential for determining the structure-property relationships in this exciting new family of solid electrolytes.