It has been demonstrated that a mixture of the solid solutions of hydrogen in a-titanium and beta-titanium may be applied as a solid-state hydrogen reference electrode in conjunction with the high-temperature proton-conducting solid electrolyte CaZr0.9In0.9O3-delta. The design and preparation of a sensor that incorporates this type of reference electrode are described, and coulometric titration experiments as well as cell voltage measurements are presented and discussed. The activity of the residual oxygen in the reference material has been identified as a critical parameter that needs to be controlled in order to ensure reliable and reproducible sensor performance. The impact of other impurities in the reference material is also considered.