Solid oxide fuel cells were tested as solid oxide electrolysis cells used for high-temperature steam electrolysis. The cells were tested at a variety of operation temperatures, current densities, and gas flows to the electrodes. The cell voltages monitored during the electrolysis operation increased significantly during the first few days of testing. Impedance spectroscopy obtained during electrolysis shows that it is the Ni/yttria-stabilized zirconia (YSZ) electrode that passivates. Reference cells and tested cells were examined in a scanning electron microscope after testing. These postmortem analyses reveal the reason for the observed passivation, because results from energy-dispersive spectroscopy clearly show evidence that silica-containing impurities have segregated to the hydrogen electrode/electrolyte interface during electrolysis testing. Examples of different microstructures and amounts of Si-containing impurities in the electrolyte/hydrogen electrode interface are presented and related to the electrolysis test conditions and the passivation histories of the electrolysis cells. (C) 2007 The Electrochemical Society.