Oxygen potential distribution profile was measured around (La,Sr)MnO 3 electrode/YSZ electrolyte interface. The local oxygen potential was measured by a newly developed microprobe that detects oxygen potential on conductive solids. A model electrode/electrolyte interface was fabricated by embedding a small piece of La 0.8 Sr 0.2 MnO 3 in YSZ. When the electrode was polarized, the oxygen potential on the electrode surface shifted out of equilibrium. The potential shift was larger and faster around the electrode/ electrolyte boundary and gradually faded with distance. The critical length was larger than 60 μm. A numerical calculation was made to simulate the relaxation behavior of the surface oxygen potential after an abrupt change in polarization voltage. The surface diffusion coefficient was roughly estimated to be about 5 x 10 -6 cm 2 s -1 .