The electrochemical membrane reactor of YSZ (yttria-stabilized zirconia) solid electrolyte coated with Pd and Ag as anode and cathode, respectively, has been applied to the partial oxidation of methane to synthesis gas (CO + H-2). The Pd\YSZ\Ag catalytic system has shown a remarkable activity for CO production at 773 K, and the selectivity to CO was quite high (96.3%) under oxygen pumping condition at 5 mA. The H-2 production strongly depended on the oxidation state of the Pd anode surface. Namely, the H-2 treatment of the Pd anode at 773 K for 1 h drastically reduced the rate of H-2 production, while air treatment enhanced the H-2 production rate. From the results of the partial oxidation of CH4 with molecular oxygen, it is considered that the reaction site of the electrochemical oxidation of CH4 to synthesis gas was the Pd-YSZ-gas-phase boundary (triple-phase boundary). In addition, it is found that the oxygen species pumped electrochemically over the Pd surface demonstrated similar activity to adsorbed oxygen over Pd, PdOad, for the selective oxidation of CH4 to CO, when the Pd supported on YSZ was used as a fixed-bed catalyst for CH4 oxidation with the adsorbed oxygen. The difference with respect to the Hz formation between the electrochemical membrane system and the fixed-bed catalyst reactor results from differences in the average particle size of Pd and the way of the oxygen supply to the Pd surface.