LaNb 0.84 W 0.16 O 4.08 has been successfully synthesized and proposed as an electrolyte for high temperature fuel cell and electrolysis applications due to its remarkable ionic conductivity. A single electrolyte-supported cell using standard electrodes has been fabricated and tested in both modes of operation, and it has been demonstrated that this material could compete with that of standard zirconia electrolytes, especially in the high temperature (HT) range. The measured current density of a non-optimized cell (~360 μm of electrolyte thickness) at 950 °C at 0.5 V (fuel cell mode) and 1.3 V (electrolysis mode) using 50%H 2 O–50% H 2 as the fuel was about − 200 mA cm −2 and about 250 mA cm −2 , respectively. The reason for the better performance in electrolysis mode is probably associated with the inherent oxygen excess of the LaNb 0.84 W 0.16 O 4.08 phase.