Thermoelectric instabilities of some refractory metal thermocouples have been studied in the temperature range 1000–2000°C under vacuum and neutral atmospheres. The sources of instability were found to be complex, involving contamination from ceramic protection tubes and furnace environments, preferential volatilization, and various thermal anomalies such as recrystallization and phase changes in the refractory thermoelements. Refractory thermocouples and thermoelements were studied under a variety of conditions and emf changes associated with each are reported. Recommendations are presented for the selection and application of these thermocouples to attain maximum thermoelectric stability.