Effects of environment on the EMF stability of a thermocouple are discussed.Stability of EMF of 0.128 and 0.032 in. diameter Type K thermocouple at 1800 F in air under laboratory conditions is presented and method of test described. The EMF change of an individual thermoelement with respect to its initial condition is compared with corresponding change in microstructure. Test data for the 0.032 in. diameter wire extends to 1000 hr while those for 0.128 in. diameter wire extends beyond 4000 hr.The stability of EMF for a temperature range of 1800-2200 F in air of a Type K thermocouple is compared with Tophel II- Nial II * in both laboratory and industrial conditions. The EMF changes of a 0.128 in. diameter Type K and a Tophel II-Nial II couple of the same size upon exposure to a reducing atmosphere at 1750 F are presented.Approaches to the design of a more stable thermocouple are discussed.