The　interfaces　between　cathode　and　electrolyte　in　solid　oxide　fuel　cells　(SOFCs)　play　a　critical　role　in　the　overall　performance　and　durability,　and　are　generally　formed　by　pre-sintering　at　high　temperatures,　e.g.,　~1150°C　in　the　case　of　La0.8Sr0.2MnO3　(LSM)　cathodes.　Here　the　interface　between　LSM　and　yttria-stabilized　zirconia　(YSZ)　or　Gd-doped　ceria　(GDC)　electrolytes　formed　under　high　temperature　sintering　is　studied　using　Focused　Ion　Beam　and　Scanning　Transmission　Electron　Microscope　(FIB-STEM)　techniques.　In　the　case　of　LSM/YSZ　interface,　there　is　a　significant　cation　interdiffusion,　particularly　Mn2+　into　YSZ　electrolyte　in　the　convex　contact　ring　region　and　Mn,　La　and　Y　cation　diffusion　along　the　grain　boundary　of　YSZ　electrolyte　also　occurs.　For　LSM/GDC　interface,　no　cation　interdiffusion　was　observed.　The　results　indicate　the　formation　of　semi-coherent　interface,　i.e.,　an　atom-by-atom　matching　only　exists　locally　in　regions　along　the　interface,　but　orientation　relationship　and　lattice　mismatch　factor　between　electrode　and　electrolyte　crystallographic　planes　at　the　LSM/YSZ　and　LSM/GDC　interfaces　vary　significantly.　Lattice　disorientation　and　misfit　of　the　crystallographic　planes　occur　within　a　narrow　region　of　0.10–0.5　nm　wide　with　no　amorphous　or　solid　solution　formation.　The　results　demonstrate　that　cation　interdiffusion　is　not　an　essential　requirement　for　the　formation　of　heterointerfaces　for　LSM　electrode,　and　disorientation　and　dislocation　of　the　crystallographic　planes　at　the　interface　does　not　impede　the　oxygen　diffusion　and　incorporation　process　for　the　O2　reduction　reaction.　©　2017　The　Electrochemical　Society.　All　rights　reserved.