The reaction of superoxide with nitrogen monoxide has been reinvestigated. Photolysis of alkaline peroxynitrite solutions results in the formation of superoxide and nitrogen monoxide, which subsequently react at a rate of (1.5 +/- 0.1) x 10(10) M-1 s(-1). When hydrogen peroxide is photolyzed in the presence of nitrogen monoxide, hydroxyl radicals are formed; these react with hydrogen peroxide to form superoxide, which then reacts with the nitrogen monoxide present. For the reaction of nitrogen monoxide with superoxide, a rate constant of (2.0 +/-0.4) x 10(10) M-1 s(-1) is derived. Photolysis of nitrite gives rise to hydroxyl radicals and nitrogen monoxide. In the presence of formate and oxygen, the hydroxyl radicals subsequently form superoxide radicals, which, in turn, react with nitrogen monoxide. From these experiments, a rate constant of (1.3 +/- 0.2) x 10(11) M-1 s(-1) is derived. We thus report a rate constant of (1.6 +/- 0.3) x 10(11) M-1 s(-1), the weighted average of these three photolysis experiments.