A method is described for determining the standard free energies of formation of the tantalum silicides from emf measurements on cells with solid thoria‐yttria electrolytes. Measurements were conducted over the temperature range 900°–1100°C in a purified argon atmosphere. Corrections for electronic conduction were established with Ta, electrodes. Difficulties were encountered because the hard, refractory nature of the compounds prevented fabrication of dense electrodes and because silica films interfered with the measurements. At 1300°K, the measured standard free energies of formation in kcal per gram atom of silicon are: , ; , ; , ; , .