AbstractIt was found that light emission produced by the oxidation of luminol by potassium ferricyanide in basic medium was enhanced by synephrine, an anti‐obesity drug. The optimum conditions for this chemiluminescent reaction were studied in detail in a flow injection system and employed in a new, simple and rapid method for the determination of synephrine. A mechanism for this reaction is proposed, based on the chemiluminescence reaction spectra. In the optimum conditions, CL intensity is proportional to concentration of synephrine in the 0.008–1 µg/mL range. The limit of detection is 1.6 ng/mL for synephrine (3σ), and the relative standard deviation (n = 11) is 2.6% for 0.5 µg/mL synephrine. The method was applied to the determination of synephrine in herbal products, citrus fruit and biological fluids. The recoveries were satisfactory (90–102%). The results given by the proposed method are in good agreement with those given by HPLC‐UV. Copyright © 2005 John Wiley & Sons, Ltd.