High temperature solid oxide cells are attractive electrochemical reactors to perform simultaneous reduction of steam and CO2 into syngas with the possibility of delivering a tailored composition to downstream processes. The main thermodynamic considerations, performance and impedance curves of electrolyte supported cells in co-electrolysis are shown as a function of temperature and pressure. In dynamic operation, the thermal changes that the reactor undergoes appear stronger in co-electrolysis mode than in steam electrolysis and should be considered in the development of operating strategies.