A mathematical model is proposed to understand the sensitivity of internal temperature measurements recorded by thermocouples during microwave heating experiments. In situations of internal heating, such as when microwaves are used, the portion of the metal thermocouple guard protruding from the material body remains relatively cool and represents an effective path for thermal loss. This suggests that the temperature recorded at the tip of the embedded portion of the thermocouple may be strongly dependent on the ability of the metal guard to transfer heat away from this tip. The analytical results derived here support this thesis.