The error of an ``immersed'' thermocouple due to heat leaks through the thermocouple wires has been estimated by means of a simple model. The error is shown to decrease exponentially with quantityL1/l1, whereL1is the ``immersion'' depth of the thermocouple andl1is a characteristic length depending on thermocouple parameters and the mode of heat transfer. It is shown that large errors generally arise if such a thermocouple is used in vacuum. Calculations for specific cases are given.