For the measurements of the partial ionic conductivities and the Mg 2+ migration barriers of the MgB2Se4 spinels, a sandwich-type cell configuration was used, already described for the investigation of MgSc2Se4 in our previous work. Inside the cells, Mg-ion conducting and at the same time electron-blocking interlayers were inserted between the cold-pressed spinel pellet and the stainless-steel ion-blocking electrodes (SS). Hence, the interlayer does not only enable a better physical contact between the different components, but also suppresses the undesired electron transport which would otherwise make the evaluation much more difficult.