4-Iodophenylboronic acid has been synthesized and shown to be a potent enhancer of the chemiluminescent horseradish peroxidase (Type VI-A)-catalyzed oxidation of luminol and isoluminol. The enhancer was effective (>100-fold enhancement) in the concentration range 10–1500 μm. Light emission in the presence of the enhancer peaked at 5–10 min after initiation of the reaction and then decayed very slowly. 4-Iodophenylboronic acid also enhanced reactions catalyzed by horseradish peroxidase Type IX. The detection limit for Type VI-A horseradish peroxidase was 509 amol, and optimum signal enhancement was obtained at 769 μmcompared to 154 μmfor 4-iodophenol under the same conditions.