A b s t r a c t. In recent years numerous studies have demonstrated the feasibility of the infrared laser photoacoustic spectroscopy (PAS) for agricultural, biological, medical and environmental applications. The photoacoustic spectroscopy is one of the most valuable analytical methods in gas detection, owing to the high sensitivity, high selectivity and the capability of in situ detection. A great number of trace gases can be detected and monitored with the photoacoustic method combined with various molecular gas lasers. These measurements can be done continuously, simultaneously over long periods and with high time resolution. In the Bonn laboratory a highly sophisticated photoacoustic detection system is available, including computer control for automatic selection of laser frequencies and adaptation of the signal detection unit to varying operation conditions of the spectrometer. The data taken during the measurement are stored, processed and displayed by the computer. These features are available for the photoacoustic spectrometer using the CO₂-laser as well as the CO-, CO-overtone- and furthermore the N₂O-Iaser. In our experiments a spectrometer consisting of either the CO₂-Iaser or the CO-overtone laser and the resonant photoacoustic cell placed intracavity has been used. For example we have reached a detection limit of 30 ppt for ethylene (C₂H₄) and 37 ppt for methane (CH₄). Our photoacoustic work profits from the collaboration with the research group of Professor J. Reuss in Nijmegen (The Netherlands). The photoacoustic detection method using the CO₂-laser as powerful source has been developed to a high degree of perfection in the Nijmegen group.