A mobile fiber-optical sensor system for the on-line and in situ detection of aquatic fluorophores has been developed. By the use of time-resolved laser-induced fluorescence spectroscopy the determination of contaminants, i.e., polycyclic aromatic hydrocarbons (PAH) or fluorescence tracers in various environments, is possible. In both cases attempts to detect these substances in water by means of fluorescence spectroscopy are complicated by the low concentrations and the overlapping and featureless fluorescence spectra in combination with background fluorescence caused by further compounds, e.g., humic material. By collecting the fluorescence decay time as an additional independent dimension, the analytical information is significantly increased, and to a certain extent the determination of the desired analyte in complex natural matrices is possible. At a first application, the detection of pyrene (PYR) in real samples from a contaminated former coking plant site has been realized. The system is also best suitable for hydrogeological research. Here applications spread from the investigation of the fluorescence tracer migration in an artificial aquifer system to the determination of hydrogeological parameter at a domestic waste disposal.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.