ABTS(2-), 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) dianion, was used as a reference to compare the reactivity of peroxyl radicals of two amino acids, glycine and valine, in aqueous solutions at natural pH. Peroxyl radicals were produced by pulse radiolysis and the product of their reaction with ABTS(2-) the ABTS(center dot-) radical was observed spectrophotometrically. Experimental kinetic traces were fitted using chemical simulation. The rate constants of reactions of glycine and valine peroxyl radicals with ABTS(2-) were (6.0 +/- 0.2) x 10(6) and (1.3 +/- 0.1) x 10(5) M-1 center dot s(-1), respectively. Moreover, it was found that only 60% of glycine radicals formed upon its reaction with (OH)-O-center dot radicals reacted with molecular oxygen to yield peroxyl radicals. Comparison of experimental data with simulations of chemical reactions in irradiated ABTS and ABTS/NaSCN solutions showed that ABTS(center dot-) forms in the reaction with (OH)-O-center dot with a yield of 43% and rate constant of (5.4 +/-0.2) x 10(9) M-1 center dot s(-1) and in the reaction with (SCN)(2)(center dot-) with a yield of 57% and rate constant of (8.0 +/-0.2) x 10(8) M-1 center dot s(-1).