Abstract: An improved hot junction construction for thermocouples is provided by means of a tight fitting internal tube positioned over the outer end of the thermocouple wire prior to its fusion with the tip tube. The internal tube is formed of an alloy, such as an iron alloy containing chromium and nickel, such that the components thereof form a metal solution with the components of the thermocouple wire, which may be of a nickel-containing alloy such as copper-nickel alloy, and the external tube formed, for example, of an iron-chromium alloy. When the outer ends of the wire, internal tube and tip tube are fused, a hot junction is produced which is substantially free of pores, gaseous inclusions and stresses.