Recent thermodynamic and electrical conductivity data are evaluated to select the most appropriate electrolyte composition for IT-SOFC operation at 500 degrees C. Ce0.9Gd0.1O1.95 has an ionic lattice conductivity of 10(-2) S cm(-1) at 500 degrees C, and the Gd3+ ion is the preferred dopant, compared to Sm3+ and Y3+, at this temperature. Thermodynamic investigations indicate that for CeO2-Re2O3 solid solutions at intermediate temperatures it becomes easier to reduce Ce4+ as the concentration of Re2O3 is increased. As the associated electron mobilities do not appear to be a strong function of composition it follows that Ce0.9Gd0.1O1.95 has a wider ionic domain than Ce0.8Gd0.2O1.9 at intermediate temperatures. particular attention is drawn to the deleterious effects of impurities (principally SiO2) which are responsible for large dopant concentration dependent grain boundary resistivities. These grain boundary resistivities can obscure the intrinsic lattice ionic conductivities and cause investigators to select non-optimal dopant compositions. It follows that the use of clean (SiO2 < 50 ppm) powders is strongly recommended, particularly as these are now commercially available. At present there is also no compelling evidence to confirm that the intrinsic lattice electronic conductivity is significantly changed by co-doping with Pr6O11. Finally the I-V characteristics of single cells incorporating 25-mu m thick Ce0.9Gd0.1O1.95 electrolytes are modelled, and the requirements for composite electrodes briefly discussed so that power densities of 0.4 W cm(-2) at 500 degrees C can be attained.