For classifying plastics, the near-infrared spectroscopy (NIRS) has already been used for several years. The present master thesis concerns about the detection and effect of external influences on various common sorts of plastic, which are being examined and analysed by NIRS. The theoretical part begins with an introduction on the topic on plastics. After an overview of the historical development, information of the examined kinds of plastics is given, followed by a digression about recycling of these materials. Afterwards the topic of sensor based sorting technology is being discussed. The general settings of a sorting stand and various types of sensors are introduced. The near infrared technique, which is applied in this master thesis is also explained. The practical part describes the experimental construction of the used sorting stand. Then superficial influences, such as dirt, moisture, fuel or the medium term effect of weather on the plastics are presented. Changes on the infrared spectrum, in a region between 1000 and 1700 nm, are recorded. In conclusion the gathered results are analysed and a resume is drawn.