Bi2O3 and Gd2O3 co-doped CeO2 is a promising electrolyte for its high electrical conductivity and low sintering temperature, but its densification behavior and the reason for the high conductivity are unclear. In this study, Ce0.9Gd0.1-xBixO1.95-delta (x=0-0.05) oxide has been synthesized by a co-precipitation method and its densification behavior and electrical conductivity are systematically investigated as the electrolyte material for solid oxide fuel cells. With the increase of Bi doping content, the sinterability of Ce0.9Gd0.1-xBixO1.95-delta is promoted. Based on the master sintering curve (MSC), the activation energy for densification process is calculated. Compared with Ce0.9Gd0.1O1.95 without Bi2O3 doping, the activation energy for the sample with Bi doping content x = 0.04 is reduced from 725 kJ mol(-1) to 450 kJ mol(-1). The bulk conductivity increases gradually with Bi2O3 doping, which can be attributed to the increase of the free volume induced by Bi3+ with larger ionic radius than that of Gd3+. However, the grain boundary conductivity changes inconspicuously with different Bi doping content, which may be associated with the similar oxygen vacancy concentration in the space charge layer. For all the samples, the grain boundary conductivity is lower than the bulk conductivity at 300 degrees C, which, according to the space charge blocking effect, can be attributed to the lower oxygen vacancy concentration in the space charge layers than that in the bulk. (C) 2015 Elsevier Ltd. All rights reserved.