X‐ray analysis and electrical conductivity measurements (800°–1000°C) on yttria‐hafnia solid solutions between 2 and 20 m/o (mole per cent) were made to locate the cubic phase and to select the composition best suited as a solid electrolyte. The phase boundary is located near 7 m/o . Total conductivities obtained from the cubic phase solid solutions appear to be ionic. The composition showed the highest conductivity , and lowest activation energy (16.9 kcal/mole), and was selected as the optimum composition for a solid electrolyte. Open‐circuit emf, electrical conductivity, and Wagner d‐c polarization measurements between 800° and 1000°C were made to determine the electrolytic domain of the composition. The electrolytic domain width at 1000°C extends from log yttria‐stabilized hafnia composition does appear suitable as a solid electrolyte. Its electrical properties are very similar to those of calcia‐stabilized zirconia.