Thermoelectric instabilities of noble metal thermocouples and individual thermoelements (platinum, rhodium, iridium, iridium‐50% rhodium, and several platinum‐rhodium alloys) have been studied in the temperature range from 1000 to 1700°C under oxidizing and neutral atmospheres. The principal source of instability was found to be contamination of the thermoelements by impurities from ceramic protection tubes. Several grades of alumina sheathing were tested, and thermocouple errors associated with each grade are reported. Criteria are discussed for the selection and application of noble metal thermocouples to attain maximum thermoelectric stability.