Early data on the aging characteristics of experimental heats of Nicrosil (Ni-14.2Cr-1.4Si-0.1Mg) and Nisil® (Ni-4.2Si-0.1Mg) versus Type K thermoelements are reviewed. The electromotive force stability of Nicrosil-Nisil in 1.6-mm (0.064-in.), 0.8-mm (0.032-in.), and 0.3-mm (0.0126-in.) diameter wire at 500°C in air up to 200 h is obtained in situ and is compared with that of Type K thermoelements of the same size. The mechanism of EMF changes is discussed.