Abstract: A fluorescently labeled complex hydrophobic fluid produced by dissolving a porphyrin therein. The porphyrin must be substantially soluble in acidic aqueous solutions, substantially insoluble in neutral or alkaline aqueous solutions and fluoresce at a wavelength between about 600 nm and 800 nm when irradiated at a wavelength between about 350 nm and 550 nm. The fluorescently labeled complex hydrophobic fluid may be identified by observation of the above-mentioned characteristic fluorescence upon irradiation. For identification purposes the porphyrin may be first extracted into an acidic aqueous solution for determination of fluorescence. In cases where the labeled hydrophobic fluid has been dispersed in a quantity of neutral or alkaline aqueous solution or suspension, this latter solution or suspension is first extracted with a hydrophobic solvent for the porphyrin to form a porphyrin solution. The prophyrin, in turn is extracted from this prophyrin solution with an acidic aqueous solution.