High-temperature co-electrolysis shows comparable performance to steam electrolysis. Current densities above 1 A cm(-2) can be reached between 700 degrees C and 800 degrees C. Tailor-made syngas is produced, mainly determined by the reactant ratio. The experimental results are supported by modeling. Durability tests with cathode-supported cells show increased voltage degradation rates during electrolysis compared to fuel cell operation. Nickel depletion is found to be the main cause.