The rate constants for the reactions of horseradish peroxidase compound I (kl) and compound I1 (k2) with three 4-substituted arylboronic acids, which enhance chemiluminescence in the horseradish peroxidase catalyzed oxidation of luminol by hydrogen peroxide, were determined at pH 8.6, total ionic strength 0.1 1 M, using stopped-flow kinetic measurements. For comparison, the rate constants of the reactions of 4-iodophenol with compounds I and I1 were also determined under the same experimental conditions. The three arylboronic acid derivatives and their rate constants are: 4-biphenylboronic acid, kl = (1.acid, which shows comparable luminescent enhancement to 4-iodophenol, has the highest reactivity in the reduction of both compounds I and I1 among the three arylboronic acid derivatives tested.