High-temperature co-electrolysis of carbon dioxide and steam is a promising method to produce 'white' syngas by making use of renewable energy and carbon dioxide as sustainable feedstock. The technological key advantage is the possibility to tailor syngas compositions over a broad range. This paper presents a systematic investigation of the syngas tailoring process by establishing relationships between feed gas compositions and flow rates to the syngas ratio. A linear dependence between the H2O:CO2 ratio in the feed gas and the H-2:CO ratio in the output gas was observed. Furthermore, the syngas ratio remains mostly invariant upon variations in electrochemical potential and fluctuating gas utilizations/flow rates during operation of a co-electrolysis cell. Most importantly, the co-electrolysis performance was demonstrated to operate at high current densities of up to 3.2 A.cm(-2) over a broad range of feed gas compositions with faradaic efficiencies of nearly 100%. The possibility to operate co-electrolysis under transient load conditions renders this method particularly suitable in future scenarios of intermittent availability of renewables. The results described here illustrate the versatility of co-electrolysis, which can produce all relevant syngas compositions in a single-step process at constantly high performance. (C) The Author(s) 2019. Published by ECS.