There have been many studies on the use of fluorescence measurements to monitor the concentrations of aromatic compounds. Recently, there are a few commercial fluorescence probes on the market that are tuned to a fixed emission fluorescence frequency. Because of the non-invasive nature, fluorescence and optical techniques are ideally suited for the on-line monitoring of aromatic compounds. A rigorous sensor model was developed for a commercial fluorescence probe to describe the single frequency excitation and emission fluorescence behavior of a mixture of fluorophores. This model is needed to correlate the measured signals to the concentration of fluorescent compounds. The relevant parameters of the model are the absorbance of the medium at both the excitation and the emission frequencies by the solvent and other absorbing species, the background signals, the light path length of the fermentor vessel, the fluorescence yield, the lamp-detector configuration, and a light-scattering coefficient. The effects of temperature, pH, and spatial inhomogeneity (bubbles and insoluble solids) are implicitly included in the model. The model shows that the relationship between the level of fluorescence signal and the concentration of the fluorophore of interest in the presence of other interferences is intrinsically nonlinear. The signal level is independent of the fluorophore concentration for the ideal case of a single fluorophore and an infinitely long light path. The validity of the model was verified experimentally with a simple system of known fluorophores and extended to a complex system containing microbial cells in an aerated fermentor.