The yield of hydroxyl radicals produced in the decomposition of peroxynitrous acid (HOONO) at room temperature in deoxygenated and bicarbonate free water at pH similar to 6.8 has been determined to be roughly 10%. This value rests on a detailed study of the decomposition of peroxynitrous acid in the presence of dimethyl sulfoxide with stopped-flow kinetics and product analyses by unequivocal methods. The HO./DMSO reaction is known to yield methane sulfinic acid (MSA) and CH3. radicals with 91% efficiency. MSA was quantified by H-1 NMR and its measured yield was corrected to allow for its extensive further oxidation to methane sulfonic acid. Methyl radicals were quantified by trapping with a water-soluble, stable nitroxide. At low peroxynitrite concentrations these two techniques gave HO. yields of ca. 8% and ca. 13%, respectively. We conclude that in water the main (ca. 90%) decomposition pathway for peroxynitrite involves a rearrangement to nitric acid via an in-cage collapse of the singlet HO./(NO2)-N-. radical pair which may, in part, be preceded by electron transfer to form an HO-/+NO2 intimate ion pair. We emphasize that, in contrast to many earlier reports, a distinct pathway to hydroxyl radicals is present, which implies that a significant portion of the oxidative nature of peroxynitrite can stem from hydroxyl radical-induced chemistry.