Doped cerias are currently being used as oxide ion conducting electrolyte materials in solid oxide fuel cells (SOFCs). Depending on the nature and concentration of the dopant and the oxygen partial pressure (pO2) these materials can also act as mixed ionic-electronic conductors (MIECs). MIECs have a particular advantage for use in SOFCs as they allow for redox reaction over the whole surface of the electrode. This potentially allows for greater efficiency in these devices. Understanding the details of vacancy distribution is a key component in the elucidation of the ionic conduction mechanism and the development of these materials. A total neutron scattering study of vacancy ordering in praseodymium doped ceria as a function pO2 is proposed. Reverse Monte Carlo analysis of total neutron scattering data uniquely affords a method of characterising vacancy ordering in such systems.