AbstractTheoretical analyses and experiments have been performed for rapid detection of dissolved humic substances in surface waters. The fluorescence has been generated by a nitrogen laser in a laboratory. An optical model has been developed, which has been applied to eutrophic waters around Berlin. The model is based on the assumption, that humic substances and phytoplankton dominate the fluorescence behaviour of the investigated waters. To verify the results of the model, regression analyses between fluorescence data of natural samples and the chemical oxygen demand of filtrates have been conducted. When green and red fluorescence values and the green fluorescence decay time were considered, a maximum squared correlation coefficient of typically 0.85 occurred.