The kinetics of the oxidn. of nitrite to nitrate by peroxynitrous acid at pH 5.2 is best described by the rate law kobs = kiso + k'[NO2-] + k''[NO2-]2, in which the peroxynitrous acid isomerization rate const. kiso = (1.10 +- 0.05) s-1, k' = (3.2 +- 0.1) M-1 s-1, and k'' = (4.2 +- 0.3) M-2s-1, at 25 DegC. The ternary reaction may involve initial formation of an adduct between nitrite and peroxynitrite, followed by reaction with a second nitrite to form two nitrite and a nitrate. Ab initio calcns. indicate that there is only a small intrinsic barrier to the net transfer of HO+ from peroxynitrous acid to the nitrogen atom of nitrite. A similar transfer to either of the two oxygens of nitrite produces the reactants, and would not lead to an increase in the rate of disappearance of peroxynitrous acid, as obsd. The low rate const. is most likely due to stringent orientational constraints. Formal transfer of HO+ to 15NO2- results in formation of 15NO3-, as exptl. obsd. HO+ transfer is common to the chem. of peracids, a family of compds. to which peroxynitrous acid belongs. [on SciFinder (R)]