The interaction of superoxide radicals with N-acetylcysteine (RSH) in an aqueous solution of pH 7 using the technique of steady state radiolysis has been investigated in this paper. The radiolytic yield of the products (G value) of RSH consumption and disulfide of N-acetylcysteine (RSSR) formation has been determined. The G value of the products is not dependent on the concentration of RSH (at the plateau of dilution curve) or on the inverse of the square root of the dose rate (dose rate)(-1/2), from which it is concluded that in this reaction there is no character of chain reaction. The disulfide of N-acetylcysteine is the only sulfur final product. Hydrogen peroxide is not a reaction product, and accordingly the reaction of O(2)(*-) with RSH does not proceed via hydrogen atom abstraction from RSH. A reaction mechanism is proposed, and an overall rate constant of 68 M(-1) s(-1) has been estimated.