Oxide ceramics based on ceria, CeO2, have been the subject of considerable research for the role of solid electrolyte within solid oxide fuel cells (SOFCs), since their ionic conductivities at temperatures around 1000K are significantly higher than the current 'best' system, yttria-stabilised zirconia. However, under reducing conditions, such as those found at the anode side of an SOFC, the reduction of Ce4+ to Ce3+ gives rise to a significant electronic component to the conductivity which is detrimental to the operation of the fuel cell. This proposal aims to characterise the structural changes associated with the reduction process, using neutron diffraction data collected on Polaris as a function of oxygen partial pressure. Specifically, analysis of the total scattering will probe the development of defect clusters as the concentration of charge compensating oxygen vacancies increases.