The aim of this study is to investigate the effect of grain size on the electrical properties of Ce0.9Gd0.1O1.95-x mol% MgO(GDC-xMgO) and to evaluate them as electrolytes for use in intermediate-temperature solid oxide fuel cells (IT-SOFCs). For this purpose, GDC-xMgO (x = 0-15)electrolytes were synthesized by the glycine-nitrate process and sintered at different temperatures. Impedance spectroscopy measurements revealed that for each composition, the grain -boundary resistivity decreased with decreasing grain size for the samples with grain size of >0.4 mu m. Much too small grain sizes (0.2 < dg < 0.3 mu m) produced an increase in grain -boundary resistivity. The addition of MgO could weaken the influence of grain sizes on the grain -boundary resistivity. The interfacial polarization resistances could be decreased by adding MgO to GDC. The GDC-1MgO sample sintered at 1200 C exhibited the highest total conductivity of 8.11 x 10(-2) S cm(-1) at 800 degrees C. The maximum power density of the GDC-1MgO-based cell was 0.73 W cm(-2) at 800 degrees C, which was much higher than that of the GDC-based cell. The results indicated that the GDC-1MgO was a potential electrolyte for IT-SOFCs. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.