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 (HO2 center dot/HO center dot > 25:1). All of the NO center dot and HO2 center dot 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 HO2 center dot and NO center dot, but after recombination, the absorbance levels are lower than initial values, possibly because HO center dot produced by the photolysis of water reacts with ONOOH. When NO3-, 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, NO3-, 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.