A method is described for the simultaneous determination of molybdenum and boron in plant leaves, based on the highly sensitive fluorescence reaction with Alizarin Red S. Simplicity is achieved by using first and second derivatives of the synchronous spectra. Appropriate calibration equations were established between 0.1–0.9 and 0.3–0.7 μg ml−1 for molybdenum and boron, respectively, with a mean relative standard deviation of 2.2%.