The performance of electrodes in direct-utilization, solid oxide fuel cells (SOFCs) has been studied on anode-supported and electrolyte-supported cells using impedance spectroscopy, coupled with calculations of the potential distribution in the electrolyte. The cells in these studies were composed of a Cu-ceria-yttria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, and a Sr-doped LaMnO3 (LSM)-YSZ cathode and were operated at 983 K using both H-2 and n-butane as fuel. Both calculations and experiments show that three-electrode measurements on anode-supported electrolytes, with the reference electrode opposite the anode, provide no additional information over two-electrode measurements and cannot be used to estimate the performance of individual electrodes. Three-electrode measurements were able to estimate anode and cathode performance on thick, electrolyte-supported cells, with symmetric placement of the working electrodes. However, both experiments and calculations demonstrate that differences in the kinetics of the two electrodes make perfect separation of anode and cathode processes difficult. The cathode performance of LSM-YSZ in these experiments was described by a single arc in the Cole-Cole plot, with a frequency of 2 kHz and a resistance of 0.4 Omega cm(2). The performance of the anode in H2 was also characterized by a single arc, with a frequency of 4 Hz and a resistance of 0.8 Omega cm(2). While anode performance in H2 is only weakly dependent on current density, nonlinear processes are observed with n-butane, so that the area-specific resistances depend strongly on the current density. (C) 2003 The Electrochemical Society.