The poisoning effect of a sulfur-containing impurity on the electrochemical oxidation of H-2 has been studied at 1023-1273 K in a gas mixture of H-2 and H2O at the interface between a Ni-YSZ cermet electrode and a YSZ electrolyte using a complex impedance analysis and a DC polarization method. The polarization resistance and the overvoltage of the electrode increased when the H2S concentration exceeded 0.05, 0.5, and 2 ppm at 1023, 1173 and 1273 K, respectively. A large temperature dependence was observed, which insists that a high-grade desulfurization is necessary at lower operating temperatures. The time needed for the influence of the sulfide impurity to saturate was almost independent of the sulfide concentration, and was found to be approximately 12, 9 and 4 ks at 1023, 1173 and 1273 K, respectively. Within the present experimental conditions, the performance loss caused by the sulfur-poisoning was recoverable when the sulfur source was removed from the fuel; the time needed for the recovery was approximately 360, 90 and 4 ks at 1023, 1173 and 1273 K, respectively. The degree of sulfur-poisoning was found to depend not on the equilibrium partial pressure of S-2 but on the total sulfur content in the fuel.