Oxide ion flux at cathode/electrolyte interface of solid oxide fuel cell (SOFC) is investigated through quenching reaction and oxygen isotope labeling. A YSZ (yttria-stabilized zirconia) electrolyte-supported cell with LSM (strontium-doped lanthanum manganite)/YSZ porous cathode is operated by supplying O-18(2) at 973 K and abruptly quenched to room temperature by a direct helium gas-impinging jet to the cell. The O-18 concentration distribution in the cross section of the cathode/electrolyte interface is obtained by secondary ion mass spectrometry (SIMS) with a spatial resolution of 50 nm. From the analysis of oxygen isotope diffusion profiles in YSZ electrolyte, oxide ion flux incorporated from a cathode/electrolyte interface to an electrolyte is first estimated. The obtained flux 1.01-1.43 x 10(-3) mol m(-2) s(-1) at a current density of 0.09A cm(-2) indicates that 22-31% of the overall electrochemical reaction occurs at the cathode/electrolyte interface, while the remaining 69-78% of those proceeds inside the porous cathode under the present experimental condition.