Yields of O-2 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)-O-. and (NO2)-N-. radicals as oxidizing intermediates. The kinetic model used includes all reactions of (OH)-O-., O-.(2)-, 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 O-2 yields and (2) the unusual effects of NO2-, which inhibits O-2 formation in neutral, but not alkaline, solutions and also reverses inhibition by organic (OH)-O-. scavengers in alkaline media. Other observations, including quenching of O-2 yields by ferrocyanide and bicarbonate, the pressure dependence of the decomposition rate, and the reported dynamic behavior for O-2 generation in the presence of H2O2, also appear to be in accord with the suggested mechanism. Overall, the close correspondence between observed and calculated O-2 yields provides strong support for decomposition via homolysis of the ONOOH peroxo bond.