Quantitative investigation is conducted on the resistance sources of the components in the NiO-YSZ/YSZ/GDC/LSCF GDC solid oxide electrolysis cell (SOEC) stack at the H2O/H-2 ratios of 70/30, 80/20 and 90/10 at 750 degrees C. The results indicate that the cell resistance accounts for 76.3-66.7% of that of the stack repeating unit (SRU), the contact resistance (CR) between the air electrode current-collecting layer (AECCL) and the interconnect accounts for 23.6-27.0%, the CR between the hydrogen electrode current-collecting layer (HECCL) and the interconnect only accounts for 23-3.2%, and the resistance of the interconnect can be neglected. Duration test of the stack is conducted at 0.8 A cm(-2) for 380 h, the cell resistance increase is found to be the major contribution of the SRU degradation (82.2% of the SRU degradation) while the air electrode CR increase and the hydrogen electrode CR increase are other two important factors. (C) 2014 Elsevier B.V. All rights reserved.