The electronic conductivity of the MgB2Se4 powders was measured using pouch cells. Pellets of 6-10 mm diameter were pressed from 150-300 mg MgB2Se4 powder under isostatic pressure of 3000 bar for 30 min. Afterwards, each pellet was separately sealed in a quartz glass ampule and sintered at 950 °C for 6 h (180 °C h -1 heating rate). Then, the sintered pellets were polished down to grit 4000 using SiC sandpaper. Au electrodes (Ø = 5.5-9 mm) with a thickness of 300 nm were vapor-deposited on both sides of the pellet by the mean of a sputter coater (tectra GmbH, Germany). The Au deposition rate was set to 0.15-0.2 nm s -1 at a pressure <10 -6 mbar. After a second sintering step at 500 °C for 6 h, the pellets were sealed under argon into pouch cells, connected with the Au electrodes to Ni current collectors. EIS and CA measurements of the symmetrical Au|MgB2Se4|Au cells were performed in climate chambers (Weisstechnik, Germany) at 25 °C. The impedance data were collected from 7 MHz to 100 mHz with an alternating current (AC) amplitude of 10 mV. The dc polarization was carried out in six steps from 0.1 V to 2.0 V with 12 h resting time per step. From the obtained resistances R, the electronic conductivities σel were calculated based on the following equation: