Ni pattern electrodes with well-defined triple phase boundary (TPB) lengths were prepared in order to investigate the reaction mechanisms at solid oxide fuel cell (SOFC) anodes. The anode microstructures were stable during thermal treatment and electrochemical measurements. Electrochemical impedance spectroscopy was used to study the influence of the overpotential, of the gas atmosphere, of the temperature, and of the pattern geometry on the electrochemical behavior of SOFC anodes. It is found that the reaction kinetics are dominated by one main process. This process is thermally activated with an activation of energy of eV. At overpotentials higher than 300 mV, a second process becomes relevant. The partial pressure of water in the fuel gas atmosphere has a catalytic effect on the anode performance, whereas variations of the patrial pressure of hydrogen in the fuel gas atmosphere have no significant influence on the electrode behavior. A model was established in order to explain the catalytic effect of water. The direct proportionality between the relaxation frequency and the TPB length suggests a TPB limitation of the anode kinetics.© 2001 The Electrochemical Society. All rights reserved.