The electrochemical reaction of solid oxide fuel cells (SOFCs) was reviewed in terms of mass and charge transports of reaction species around the electrode/electrolyte interfaces. Oxygen reduction and fuel oxidation were analyzed by isotope labeling and secondary ion mass spectrometry as well as conventional electrochemical method. The SIMS images after 18O2 (stable isotope oxygen) labeling suggested that the O2/cathode/electrolyte interfaces were the most active parts for oxygen reduction and incorporation. The widths of active parts of oxygen reduction were about several 100 to some 1000 nm different depending on the cathode materials and reaction mechanism. The isotope labeling-SIMS technique was also applied to visualize the active parts for CH4 decomposition and carbon deposition around the anode metal/electrolyte oxide interfaces. The active parts for carbon deposition were only on the Ni surface on YSZ electrolyte. The effect of substrate oxide on the carbon deposition was also examined at the mesh-shaped metal/oxide interfaces.