The error due to leakage current flowing through the electrical insulation between the wires of a thermocouple is calculated. The error is due to IR drop along the thermocouple wires, and increases rapidly with temperature and length, and with closeness of contact between the wires and solid insulation. Thus, temperature measurements by swaged sheathed thermocouples in large, high-temperature, liquid-metal and gas-cooled nuclear reactors and heat transfer loops are particularly susceptible. Charts giving the error in the temperature measurements are presented for step and ramp temperature distributions along unsheathed thermocouples, and along sheathed thermocouples with either a grounded or an ungrounded hot junction. Calculation of the correction is illustrated by a typical installation.