UO2+x. and Y2O3 form fluorite-type solid solutions (U2Y1-z)O2+x, which show promise as electrode materials for high-temperature fuel or electrolysis cells. The impedance dispersion behaviour for one such composition has been studied in cells of the type (U0.7Y0.3)02+x/YSZ/(U0.7Y0.3)O2+x, in air, at temperatures between 500 and 900°C, and in the frequency range 5 Hz-500 kHz. The semi-circular arc due to electrode dispersion in the complex impedance plane is depressed substantially below the real axis. Simulation calculations show such behaviour to be due to a strong frequency dependence of the resistor(s) in the equivalent circuit for the electron-transfer reaction. We attribute such behaviour to a complex series of reaction steps at the electrode/electrolyte interface.