NOVELTY - An apparatus (10) for determining the thermal conductivity of a fluid comprises a feed unit (14) through which is a fluid is fed to a main body (12) which is associated with temperature sensor (24) for measuring temperature of main body; and a temperature control element (22) for adjusting the temperature of the main body, where a further temperature sensor for measuring the temperature of the fluid in the feed unit for detecting the presence of a liquid phase in the fluid.    USE - As apparatus for determining thermal conductivity of a fluid e.g. service fluid for fuel cell (claimed).    ADVANTAGE - The main body is constructed such that the fluid flows through it. In this way, a particularly large contact surface is provided between the fluid and the main body, which enables particularly quick and efficient heat transfer between the fluid and the main body and temperature sensor for measuring the temperature of the main body does not have any direct contact with the fluid. The main body has a thermal conductivity which is greater than the thermal conductivity of the feeder. In this way, the amount of heat lost from the feeder into the area surrounding the apparatus is kept low, while good heat transfer is made possible in the main body. In this way, the temperature control element establishes a comparatively small difference between the temperature of the main body and the temperature of the fluid in the feeder. The main body comprise a metallic material, in particular an aluminum alloy and/or a ceramic material which enables efficient heat transfer between the main body and the fluid fed.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for determining the thermal conductivity of a fluid, particularly service fluid for fuel cell involving adjusting a temperature of a main body by a temperature control element; measuring the temperature of the main body by a temperature sensor; measuring the temperature of the fluid in a feed by a further temperature sensor; feeding the fluid to the main body by the feed; to detect the presence of a liquid phase in the fluid; remeasuring the temperature of the main body by the temperature sensor.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of apparatus.    Apparatus (10)    Main body (12)    Feed unit (14)    Temperature control element (22)    Temperature sensors (24, 26)