The present paper investigates the variation in performances in steam electrolysis and co-electrolysis of steam/CO2 on SOEC single cells.In electrolysis mode, a sensitivity analysis was performed looking in particular at the effects of temperature (750°C and 800°C) and inlet gas flows (from 100 to 200 Nml/min) on the performances. I-V characterization and EIS measurements at different current densities were carried out for each sequence. As expected, the increase of temperature has a positive impact on the performances, while the change of fuel flow slightly affects them. In co-electrolysis mode CO2 and H2O content was varied, keeping constant the total molar flow and the hydrogen fraction. Resulting plots demonstrate different ongoing processes favored at high current densities and high inlet CO2 molar fraction. In particular, reverse water-gas-shift reaction plays an important role on the overall balance of the gas species, and it directly depends on temperature and inlet fuel composition.