A new solid-state hydrogen reference electrode has been developed that is based on the two-component two-phase mixture of beta-zirconium and delta-zirconium hydride, and is suitable for use in conjunction with the high-temperature proton-conducting CaZr(0.9)In(0.1)O(3-delta) solid electrolyte. Coulometric titration studies have confirmed the presence of a true two-phase plateau, existing over a wide composition range, which may be exploited as a precision thermodynamic buffer for reference hydrogen partial pressure. Cell voltage measurements have demonstrated that potentiometric hydrogen sensors incorporating this reference electrode exhibit Nernstian response over a broad range of temperature and hydrogen partial pressure, as well as excellent thermal cycling and long-term stability. The new solid-state hydrogen reference electrode is of considerable technological relevance and has already found application in a commercialised sensor unit.