The infrared technology for ACC sensors has reached a high standard up til now, but there are still problems with the detection of some vehicles. The most important factors for low detection ranges are bad weather conditions and the state of the vehicle. In order to guarantee a reliable function of the sensor it's important to know about the reflection properties of the vehicles. In this diploma work the influence of different factors on the reflection properties of vehicles was investigated. The parameters, which were taken into regard, are the colour of vehicles, the geometry of the vehicle rear, the viewing angle and the contamination of vehicles.The measurements were made with a modified LEICA ODIN 3 sensor with a very small aperture of the laser beam. This modified sensor was mounted in two axes rotatable, that the rear of the cars could be scanned, to measure the reflected radiation in every point of the rear. In this way the distribution of the reflected energy was investigated. A ODIN 4 sensor with the standard aperture of 3x3° was used to measure the total reflected energy.The most important parts of the cars refering to the reflection of infrared light are the two retroreflectors which are integrated in the taillights of the vehicles. On an average every retroreflector reflects about 40 % of the total reflected energy, that means together they reflect about 80 % of the whole energy. 18 % of the reflected radiation comes from the reflectorized licence plate and the remaining 2% from the other components of the vehicle rear like the lacquer and the rear window. The influence of the vehicle colour, the geometry of the vehicle rear and the viewing angle are low. Only the contamination of the of the vehicles leads to considerable energy loss. The maximum loss of a dirty vehicle is up to 17 dB that means that only 2 % of the energy of the clean vehicle is reflected.