Tungsten‐rhenium thermocouple systems were evaluated for use in measuring temperatures between 1600 and 3000°C. Temperature‐millivolt relationships were extended from 2300 to 3000°C for bare‐wire W3Re/W25Re and W5Re/W26Re thermocouples in vacuum. The performance of W26Re sheathed, high‐fired beryllia‐insulated thermocouples was limited only by the melting point of the beryllia insulation (∼2550°C). The thermoelectric output of high‐fired thoria‐insulated thermocouples sheathed in W26Re was determined and found to be reliable up to at least 2850°C, when corrected for electrical shunting through the insulation. Stability of high temperature thermocouples using thoria insulation was within ±0.3 mV (±40°C) when held at 2425°C for 148 h.