By using electricity from renewable sources, high-temperature solid oxide co-electrolysis cells (SOCs) can perform advantageous conversion of H2O/CO2 to high-value syngas. In this work, we investigated the performance of tubular solid oxide co-electrolysis cells for the production of syngas by electrochemical conversion of H2O/CO2. The tubular solid-oxide electrolysis cells comprise Ni-yttria stabilized zirconia (Ni-YSZ) based fuel-electrode supported cells, a yttria or scandia-stabilized zirconia (YSZ and ScSZ) electrolyte, and a composite air-electrode of (La0.85Sr0.15)(0.9) MnO3 (LSM) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF). The electrochemical performance of the tubular SOCs for various operating conditions was analyzed using I-V curves, EIS analysis, and gas chromatography. From the results, we confirm the correlation between the operating conditions and the electrochemical performance of the co-electrolysis process in the tubular SOCs. Furthermore, we found that the syngas yield of the ScSZ electrolyte-based SOC cell was better than that of the YSZ electrolyte-based SOC. The results show that using a tubular SOC offered highly efficient conversion of H2O/CO2, with high yield and good-quality syngas.