(CGO) and (ScSZ) have been proposed as possible alternative electrolytes in intermediate-temperature solid oxide fuel cells (SOFCs). In this study, the mechanisms of densely sintering Co-doped CGO and ScSZ electrolytes during the SOFC fabrication process were investigated using synchrotron X-ray diffraction (SR-XRD) analysis. The addition of CoO enhanced the sintering characteristics of both CGO and ScSZ. Based on the results of the SR-XRD analysis, it was found that CGO and CoO did not form a solid solution after heat treatment at for 10 h. On the other hand, the solubility limit of Co in ScSZ was estimated to be after firing at , and Co doping accelerated the conversion of the two phases of the fluorite structures with cubic and rhombohedral phases into a single cubic phase. Because no significant densification of the Co-doped ScSZ samples was observed before and after the phase change and Co diffusion, it suggests that these reaction sintering processes should not be strongly related to densification. From the results of scanning electron microscopy, Co doping suggests to assist the densification of the ScSZ samples through liquid phase sintering, similar to Co-doped CGO.