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Recent thermodynamic calculations of Merenyi and Lind [(1997) Chem. Re s. Toxicol. 10, 1216-1220] suggest that O=NOOH can undergo homolysis t o form the hydroxyl radical and nitrogen dioxide. This result is based in part on our statement that the enthalpy of ionization of O=NOOH is close to zero [Koppenol et al. (1992) Chem. Res. Toxicol. 5, 834-842] . As the ionization of O=NOOH is sensitive to the milieu and the rate of isomerization (to nitrate) to the total concentration of O=NOOH and O=NOO- [Kissner et al. (1997) Chem. Res. Toxicol. 10, 1285-1292], we reinvestigated the temperature dependence of the ionization constant a nd determined a Delta H degrees of 4 +/- 2 kcal mol(-1) This results i n a standard Gibbs energy of homolysis of 16 kcal mol(-1) and a rate o f homolysis of 1 x 10(-2) s(-1). Given the uncertainty in the Gibbs en ergy of homolysis, upper and lower rates are 1 x 10(-4) and 0.6 s(-1), slower than the rate of isomerization, 1.2 s(-1) at 25 degrees C. The recombination of the homolysis products NO2 . and HO . is known to le ad to mainly peroxynitrous acid. If one assumes that a few percent of the recombinations lead to nitrate instead, then the rate of homolysis must be much higher than the rate of isomerization. We conclude there fore that homolysis is unlikely.