Phenolic non-flavonoid compounds in diets, such as chlorogenic acid and caffeic acid are widely recognized to be antioxidants. However, it is not known how these phenolics scavenge reactive species of oxygen and nitrogen. We determined the rate constants of the reactions between the phenolics with superoxide and hydroxyl radical with a pulse radiolysis. The second-order rate constants of the reactions of chlorogenic acid with superoxide and hydroxyl radical were 1.67 +/- 0.14 x 10(6) M(-1) s(-1) and 3.34 +/- 0.19 x 10(9) M(-1) s(-1), respectively, while those of caffeic acid with superoxide and hydroxyl radical were 0.96 +/- 0.01 x 10(6) M(-1) s(-1) and 3.24 +/- 0.12 x 10(9) M(-1) s(-1), respectively. By scavenging peroxy radical chlorogenic acid inhibited the initiation of chain lipid peroxidations by organic free radical. The second-order rate constant of the reaction of chlorogenic acid with peroxy radical was estimated to be 1.28 +/- 0.11 x 10(5) M(-1) s(-1). Chlorogenic acid was rapidly oxidized by peroxynitrite in concentration- and pH-dependent manners and its rate constant was determined to be 1.6 +/- 0.7 x 10(5) M(-1) s(-1), using competitive inhibitions by glutathione and methionine.