In a systematic study, two compositional series of ceria-based oxides, both co-doped with Sm and Gd, were synthesised using a low temperature method and evaluated as oxygen ion-conducting electrolytes for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Series one, Ce_1-2xSm_xGd_xO_2-x, had equal concentrations of Sm and Gd but varying total dopant concentration. Series two, Ce_0.825Sm_xGd_0.175-xO_1.9125, had a fixed total dopant concentration but the Sm:Gd concentration ratio was varied. The materials were characterised using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry and X-ray diffraction. Impedance spectra were recorded on dense pellets of these materials. From these, total, bulk and grain boundary conductivities and capacitances along with activation energies, pre-exponential constants and enthalpies of ion migration and defect association were obtained. These gave a detailed insight into the fundamental conduction processes in the materials. Ce_0.825Sm_0.0875Gd_0.0875O_1.9125 had the highest total ionic conductivity at temperatures of 550 °C and above and also demonstrated an enhanced conductivity with respect to its singly-doped parent compounds, Ce_0.825Sm_0.175O_1.9125 and Ce_0.825Gd_0.175O_1.9125, at 400 °C and above. This compares favourably with previously-reported values and has promising implications for the development of IT-SOFCs.