Manganese-doped ceria-based oxides, Ce1-xMnxO2-delta (0.05 <= x <= 0.3) and Ce1-x-yGdxMnyO2-delta (0.05 <= x <= 0.2, 0.05 <= y <= 0.25) were synthesized, and crystal phase analysis by XRD and measurements of electrical properties were performed. Solubility limit of Mn in Ce1-xMnxO2-delta seemed to be between 5 mol% and 10 mol% and Mn3O4 Was the main by-product above the solubility limit in the case of heat treatment at 1300 degrees C. Judging from the oxygen partial pressure dependence of total conductivity and emf measurements, Ce1-xMnxO2-delta is a single-phase mixed conductor within the composition below the solubility limit, and when the composition of Mn exceeds the solubility limit, it becomes the dual-phase mixed conductor of Ce1-xMnxO2-delta and Mn3O4. The doing of Mn in gadlia-doped ceria, Ce1-x-yGdxMnyO2-delta (0.05 <= x <= 0.2, 0.05 <= y <= 0.25), was more difficult than that in CeO2 presumably due to the preferential reaction between Gd and Mn to give GdMnO3 to the GDC solid solution formation, and the Mn doping seems not to be so effective in preparing the mixed ionic-electronic conductor based on GDC. (c) 2006 Elsevier B.V. All rights reserved.