Stabilized bismuth oxides exhibit a decay in conductivity when annealed at temperatures below 600-degrees-C. We refer to this phenomenon as aging and it is distinct from a conventional crystallographic phase transformation. This phenomenon is revealed by an endotherm from DSC thermal analysis and results in the formation of a superstructure observable by TEM diffraction patterns, yet no change in structure is observable by XRD. Since oxygen vacancies are the mobile defects responsible for ionic conductivity, we attribute the aging process to the ordering of oxygen vacancies by an order-disorder transition below approximately 600-degrees-C.