Abstract: The electrolytic domain boundary (EDB) of doped ceria between ionic and electronic conduction is theoretically investigated with the consideration of defect association. Data from the early reports on EDB of singly, double and triply doped ceria are summarized and interpreted based on the theoretical investigation. It is proposed that samarium and gadolinium among the rare earth elements are mostly suitable for the main dopants of ceria, while further minor additions of other elements can benefit the improvement of the EDB.