High-temperature gas reactor experiments create unique challenges for thermocouple-based temperature measurements. High-temperature industrial thermocouples suffer rapid decalibration due to transmutation of the thermoelements from neutron absorption. For lower temperature applications, Type K and Type N thermocouples are affected by neutron irradiation only to a limited extent. But until recently, the use of these nickel-based thermocouples was limited when the temperature exceeds 1050°C due to drift related to phenomena other than nuclear irradiation. Certain portions of the final Advanced Gas Reactor test (AGR-5/6/7) will experience temperatures higher than any of the previous AGR tests, up to 1450°C. Recognizing the limitations of existing thermometry to measure such high temperatures, the sponsor of the AGR-5/6/7 test supported a development and testing program for thermocouples capable of low-drift operation at temperatures above 1100°C. This program included additional development of high-temperature irradiation-resistant thermocouples (HTIR-TCs) based on molybdenum/niobium thermoelements, which have been studied at INL since circa 2004. A step change in accuracy and long-term stability of this thermocouple type has been achieved as part of the AGR-5/6/7 thermometry development program. Additionally, long term testing (7000+ hrs) at 1250°C of Type N thermocouples utilizing a customized sheath developed at the University of Cambridge has been completed with excellent low-drift results. The results of this testing as well as testing of the improved HTIR design are reported herein. Both the improved HTIR and the Cambridge Type N thermocouple types have been incorporated into the AGR-5/6/7 test, which began irradiation February 2018.