Oxide ion conductors are key components in a number of technologically important applications, including oxygen sensors and pumps, membranes for oxygen separation and solid oxide fuel cells (SOFCs). In the latter case, they act as electrolytes transporting O2- to react with a fuel such as H2 in the direct conversion of chemical to electrical energy. Better understanding of why such materials show this unusual behaviour will lead to new oxide ion conductors and more efficient fuel cells operating at lower temperatures. This would have significant technological and environmental impact.A key factor for the advancement of the rational design of SOFC materials is the nature of oxide ion transport in the solid state, especially in structurally complex oxides, in which the mechanisms of O2- transport are more complex than conventional vacancy-hopping. In this proposal we request IN5 beam time to perform variable temperature, vibrational density-of-states and QENS measurements on two such oxide ion conductors: apatite-type La9.5(Ge5.5Al0.5O24)O2 and fluorite-type Bi0.913V0.087O1.587.