Laser flash photolysis of ONOOH at 355 nm and a pH of 4.0-5.5 causes homolysis of ONOOH nearly exclusively at the N-O bond rather than at the O-O bond (HO(2)(*)/HO(*) > 25:1). All of the NO(*) and HO(2)(*) radicals formed by photolysis subsequently recombine with a rate constant of (1.2 +/- 0.2) x 10(10) M(-1) s(-1) via second-order kinetics, as demonstrated by the return of the UV/vis absorbance to initial levels. Excitation at 266 nm also yields HO(2)(*) and NO(*), but after recombination, the absorbance levels are lower than initial values, possibly because HO(*) produced by the photolysis of water reacts with ONOOH. When NO(3)(-), the product of the ONOOH isomerization, is photolyzed, the ONOO(-) formed is rapidly protonated with a second-order rate constant of (1.7 +/- 0.8) x 10(10) M(-1) s(-1). The ONOOH decays to the starting material, NO(3)(-), with a first-order rate constant of 1.2 s(-1). The quantum yield for the photon-initiated homolysis is 15% for both ONOOH and ONOO(-). We conclude that the ON-OOH and ON-OO(-) bond dissociation energies are similar.