An efficient spectroiodometric method is described for the quantification of peroxynitric acid, HOONO2, and H2O2 in dilute solutions. HOONO2 reacts with I- in acidic media with a rate constant of 1200 +/- 306 M-1 s(-1). At 22 degrees C and pH 1.7, in the presence of 1.00 mM Na(2)EDTA, the average lifetime for HOONO2 thermal decay was found to be 49 +/- 26 min, longer than reported in previous work. The addition of Na(2)EDTA produced longer lifetimes, the longest one observed being 73.9 +/- 0.9 min. CU2+ was found to catalyze HOONO2 decomposition. In the presence of large concentrations of CuSO4, the decomposition rate is independent of CuSO4 and is controlled by the rate of the dissociation reaction; HOONO2 --> HO2 + NO2. For this reaction, we found a rate constant of (5.27 +/- 0.15) x 10(17) s(-1) exp(-110.1 +/- 1.3 kJ mol(-1)/RT) from 5.0 to 25.0 degrees C and an equilibrium constant of (1.5 +/- 0.2) x 10(-11) M at 25 degrees C, with an estimated reaction enthalpy of 87 +/- 3 kJ mol(-1). With the use of auxiliary thermodynamic data, we find that the standard heat of formation of HOONO2(aq) at 25 degrees C is -111 +/- 2 kJ mol(-1) and that the standard free energy of formation is 5.5 +/- 1.5 kT mol(-1). The Henry's Law constant is 12600 +/- 4700 M atm(-1), with an enthalpy of solution of -57.1 +/- 1.5 kJ mol(-1). The decomposition of HOONO2 is base-catalyzed; between 5.0 and 25.0 degrees C the decay rate constant is (1.16 +/- 0.20) x 10(16) M s(-1) [H+](-1) exp(-126 +/- 8 kJ mol(-1)/RT).