A novel type of glass electrode has been constructed in which the usual combination of internal buffer solution and reference electrode is substituted by the solid mediator substance Na+-containing Prussian blue (NaPB) (NaFeFeII)-Fe-III(CN)(6). This mixed-valent compound exhibits ionic conduction (by sodium ions) as well as electronic conduction and thus causes the potential differences at the interphases glass/NaPB and NaPB/metal to be thermodynamically well defined. Hence, with this solid-state glass electrode exceedingly stable potentiometric signals are obtained.