The reaction rates of several aminothiol, amidothiol, and phenolic antioxidants with ABTS radical cation were measured. Most compounds had half-lives of less than one minute. However several compounds had considerably longer half-lives. Aminothiol derivatives lacking a free thiol group, such as amifostine and RibCys, displayed longer half-lives. Reaction of these compounds with the ABTS radical cation displayed first order kinetic behavior. Of the phenolic compounds studied, chlorogenic acid and caffeic acid had the longest half-lives. Most phenolics displayed a biphasic kinetic pattern involving fast and slow steps. Some of the aminothiols also displayed this type of behavior. Glutathione disulfide was reactive toward ABTS radical cation and displayed slow kinetics. This suggests that the slow step observed with some of the aminothiols may be due to initial rapid formation of disulfide followed by slow reaction of the disulfide with ABTS radical cation. Some compounds required a considerably longer incubation time to reach end point than the six to ten minute period normally used for this assay. This suggests that, when ABTS is being used as an end point assay, a longer incubation time may be needed to obtain reliable data. When food substances are being tested using this assay, kinetic profiles should first be examined before end points are determined. This paper contains the first published data reporting antioxidant capacities of amino- and amidothiols measured by the ABTS method.