Molybdenum based-oxides have recently attracted interest for use as electrolyte and electrode materials in Solid Oxide Fuel Cells. This proposal requests 5 days of beamtime on the Polaris diffractometer to probe the structural properties of selected compounds in the Nd2O3-MoO3 and Tb2O3-MoO3 systems. Recent literature reports, supported by preliminary XRD and conductivity performed by the applicants, suggest that these materials possess relatively high conductivities and adopt crystal structures related to the cubic fluorite (MO2) arrangement. However, both systems are non-stoichiometric with (possibly) anion-excess and anion-deficiency, respectively. Total scattering measurements, exploiting the enhanced capabilities of the upgraded Polaris and analysed using Reverse Monte Carlo methods, will be used to probe the local disorder and correlate this with the bulk conductivity.