Yields of O₂ formed during decomposition of peroxynitrous acid (ONOOH) under widely varying medium conditions are compared to predictions based upon the assumption that the reaction involves formation of discrete ^•OH and ^•NO₂ radicals as oxidizing intermediates. The kinetic model used includes all reactions of ^•OH, ^•O₂^-, and reactive nitrogen species known to be important under the prevailing conditions; because the rate constants for all of these reactions have been independently measured, the calculations contain no adjustable fitting parameters. The model quantitatively accounts for (1) the complex pH dependence of the O₂ yields and (2) the unusual effects of NO₂^-, which inhibits O₂ formation in neutral, but not alkaline, solutions and also reverses inhibition by organic ^•OH scavengers in alkaline media. Other observations, including quenching of O₂ yields by ferrocyanide and bicarbonate, the pressure dependence of the decomposition rate, and the reported dynamic behavior for O₂ generation in the presence of H₂O₂, also appear to be in accord with the suggested mechanism. Overall, the close correspondence between observed and calculated O₂ yields provides strong support for decomposition via homolysis of the ONOOH peroxo bond.