Ce0.9Gd0.1O1.95 with various Mg doping contents was synthesized by citric acid-nitrate low temperature combustion process and sintered under different conditions. The crystal structures, microstructures and electrical properties were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and ac impedance spectroscopy. Low solubility of Mg2+ in Ce0.9Gd0.1O1.95 lattice was evidenced by XRD and FESEM micrographs. The samples sintered at 1300 degrees C exhibited the higher total conductivity than those sintered at 1100 and 1500 degrees C, with the maximum value of 1.48 x 10(-2) S cm(-1) (measured at 600 degrees C) at the Mg doping content of 6 rnol%, corresponding to the minimum total activation energy (E-tol) of 0.84 eV (150-400 degrees C). The effect of Mg doping on the electrical conductivity was signiflcant particularly at higher sintering temperatures. At the sintering temperature of 1500 degrees C, the addition of Mg (10 mol%) enhanced the grain boundary conductivity by over 101 times comparing with that of uncloped Ce0.9Gd0.1O1.95, which may be explained by the optimization of space charge layer due to the segregation of Mg2+ to the grain boundaries. (C) 2008 Elsevier B.V. All rights reserved.