An experimental program is under way to assess the performance of solid-oxide cells operating in the steam electrolysis mode for hydrogen production in a temperature range from 800 to 900°C. This temperature range is consistent with the planned coolant outlet temperature range of advanced nuclear reactors. Results were obtained from two multiple-cell planar electrolysis stacks with an active area of 64 cm2 per cell. The electrolysis cells are electrolyte-supported, with scandia-stabilized zirconia electrolytes (~140 m thick), nickel-cermet steam/hydrogen electrodes, and manganite oxygen-side electrodes. The metallic interconnect plates are fabricated from ferritic stainless steel. The experiments were performed in a range of steam inlet mole fractions (0.1 to 0.6), gas flow rates (1000 to 4000 standard cubic centimeters per minute), and current densities (0 to 0.38 A/cm2). Steam consumption rates associated with electrolysis were measured directly using inlet and outlet dewpoint instrumentation. Cell operating potentials and cell current were varied using a programmable power supply. Values of area-specific resistance and stack internal temperatures are presented as a function of current density. Initial stack-average area-specific resistance values <1.5 cm2 were observed. Hydrogen production rates in excess of 200 normal liters per hour (NL/h) were demonstrated. Internal stack temperature measurements revealed a net cooling effect for operating voltages between the open-cell potential and the thermal neutral voltage. These temperature measurements agreed very favorably with computational fluid dynamics predictions. A continuous long-duration test was run for 1000 h with a mean hydrogen production rate of 177 NL/h. Some performance degradation was noted during the long test. Stack performance is shown to be dependent on inlet steam flow rate.