The solid solution compound Li9SiAlO8 represents a single-phase Li4SiO4 derivative formally composed of 50 atom percent (a/o) Li4SiO4 and 50 a/o Li5AlO4. It exhibits an ionic conductivity sigma(Li+) = 2.3 x 10(-1) S cm(-1) at 25 degrees C. The total electronic conductivity in contact with elemental lithium or highly positively polarized electrodes is found to be around four orders of magnitude smaller. Detailed studies under different conditions and in different cell types are presented. In the asymmetric cell type (-) LilLi(9)SiAlO(8)lTa (+), the tantalum electrode served as an arbitrary positive electrode simulating different lithium activities due to the applied external voltage. Upon extreme polarization (E vs. Li/Li+ less than or equal to 6.5 V) below 100 degrees C, stable tarnishing layers were formed at the Li9SiAlO8-electrode interfaces, thus resisting further decomposition, which makes Li9SiAlO8 a suitable electrolyte for investigating cathodes which exhibit very high positive voltages vs. lithium. Moreover, chemical stability experiments show this electrolyte to be inert against elemental lithium and Li0.9Mn2O4 up to at least 100 and 500 degrees C, respectively. Li9SiAlO8 crystallizes in a monoclinic unit cell having the parameters a = 5.108(4) Angstrom, b = 5.388(4) Angstrom, c = 6.205(6) Angstrom, and beta = 90.40(9)degrees.