Abstract: An interfacial sensor and method for determining the thickness of an interface above a surface of a body around which flow occurs. The sensor has a first device for determining a first temperature, a second device for determining a second temperature and a third device for determining a third temperature. Each device is arranged at a predefinable distance (XI, X2, X3) from the surface of the body) around which flow occurs. At least the second device for determining the second temperature and the third device for determining the third temperature includes at least one wire which extends from the surface into a half-space adjoining the surface and which has a diameter of approximately 300 ?m or less. Such a sensor may find use in a wind turbine, a vehicle, an aircraft, a room climate measuring device or a ship.