Steam electrolysis over 11000 h(1) with an electrolyte-supported solid oxide cell is discussed. The cell of 45 cm(2) area consists of a scandia/ceria doped zirconia electrolyte (6Sc1CeSZ), CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel steam/hydrogen electrode. After initial 2500 h operation with lower current-density magnitude, the current density was set to j = -0.9 A cm(2) and the steam conversion rate to 51%. This led to a cell voltage of 1.185 V at 847 degrees C cell temperature. Average voltage degradation was 7.3 mV/1000 h (<0.6%/1000 h), the increase in the area specific resistance was 8 m Omega cm(2)/1000 h, sufficiently low for application in practical electrolysers. The electrical-to-chemical energy-conversion efficiency was eta(el) > 100% throughout the test (with an external heat source for evaporation). Impedance spectroscopic measurements revealed a degradation almost entirely due to increasing ohmic resistance. The rate of resistance increase was initially faster (up to 40 mn cm(2)/1000 h) and stabilised after several 1000 h operation. After 9000 h a small (non-ohmic) electrode degradation became detectable (<2 mV/1000 h), superimposed to ohmic degradation. The small electrode degradation is understood as indication for largely reversible (electrolysis cell/fuel cell) behaviour. (C) 2015 Elsevier Ltd. All rights reserved.