This paper describes a study of the high-temperature performance of nickel-chromium-silicon/nickel-silicon-magnesium (Nicrosil/Nisil or Type N) thermocouples, sheathed in alloys similar in composition to Nicrosil. Thermocouples from two sources were tested for stability under continuous exposure to progressively higher temperatures within the range 1000 to 1300°C, and for stability under temperature cycling within this range. Inconel-sheathed Type K thermocouples were included for comparison purposes, and immersion tests were also made.