A high-temperature galvanic sensor for strontium has been developed consisting of a closed-end tube of Sr-beta alumina or Sr-beta " alumina containing a reference electrode consisting of a molten SrCl2-AgCl solution in contact with a Ag wire. Measurements of the following cell are reported : Ag; (AgCl-SrCl2) Sr-beta or Sr-beta " alumina I (AgCl-SrCl2); Ag with different compositions of the salt solutions in the two electrodes. The cells are reversible, respond rapidly to temperature changes, and give emfs stable over several hours. Absolute reproducibility of +/-2 mV is obtained, and results agree with the theoretical emf calculated thermodynamically. In order to obtain the thermodynamic properties of the liquid AgCl-SrCl2 solutions required for the calculation of the theoretical emf, a critical evaluation and optimization of the thermodynamic properties and phase diagram of the AgCl-SrCl2 system was carried out.