In this study, we show that boronates, a class of synthetic organic compounds, react rapidly and stoichiometrically with peroxynitrite (ONOO(-)) to form stable hydroxy derivatives as major products. Using a stopped-flow kinetic technique, we measured the second-order rate constants for the reaction with ONOO(-), hypochlorous acid (HOCl), and hydrogen peroxide (H(2)O(2)) and found that ONOO(-) reacts with 4-acetylphenylboronic acid nearly a million times (k=1.6x10(6) M(-1) s(-1)) faster than does H(2)O(2) (k=2.2 M(-1) s(-1)) and over 200 times faster than does HOCl (k=6.2x10(3) M(-1) s(-1)). Nitric oxide and superoxide together, but not alone, oxidized boronates to the same phenolic products. Similar reaction profiles were obtained with other boronates. Results from this study may be helpful in developing a novel class of fluorescent probes for the detection and imaging of ONOO(-) formed in cellular and cell-free systems.