A model was developed which predicts the fluorescence signal measured by a commercial fluorescence probe which employs the back-scatter configuration, the signal being a function of the concentrations of fluorescing and absorbing components within the sample as well as the path length. The relationship is nonlinear in both the fluorophore concentration and the path length, due to inner-filter effects. An important parameter in the model is the exponential constant, S, which accounts for the variation in monitoring efflciency throughout the sample. The model prediction was verified for a single-component solution in which both the fluorophore concentration and path length were varied. An absorbance correction was developed which can be used to linearize the fluorescence relationship. Some aspects of multiplecomponent fluorescence are discussed as well as the expected effects of some environmental variables.