Two systematic electrolytes of Ce1-xGdxO2-x (GDC) and Ce1-xSmxO2-x (SDC) (x = 0-0.25) were synthesized using an oxalate coprecipitation process. Dependence of a, unit cell parameter versus dopant concentration, x, of Gd3+ and Sm3+ ions show that these solid solutions obey Vegard's rule as a = 5.4121 + 0.0525x for GDC and a = 5.4117 + 0.1237x for SDC, respectively. Electrical conductivity reached maximum at x = 0.15 in the temperature range of 400-850 degreesC for both kinds of doped-ceria electrolyte membranes. A single cell was made for the measurement of open circuit voltage. The results show that the open circuit voltages are greatly influenced by ionic transference number of the electrolyte, gaseous fuel composition and cathode membrane material.