High purity cerium and gadolinium salts were used to form ceria-based solid solution (Ce1-xGdxO2-delta, 0.05less than or equal toxless than or equal to0.4) through the oxalate coprecipitation. Crystal structure and microstructure were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The ionic conductivity (i.e., lattice, grain boundary (GB) and total conductivities) in this system were systematically studied as a function of dopant content over the temperature range from 200 to 850 degreesC in air using an impedance spectroscopy. The grain boundary conductivity increases rapidly at x<0.1 and is almost unchanged between x=0.1 and x=0.2, then decreases at x>0.2. The Gd content has a slighter influence on the lattice conductivity in the Gd content ranging from 0.05 to 0.2, while a rapid decrease in the lattice conductivity is observed at x>0.2. A maximum total conductivity is observed for the composition x=0.2.