The formation of ONOOH/ONOO- in water has been studied using the pulse radiolysis of nitrite and nitrate solutions. The overall rate constant of the reaction of (OH)-O-. with (NO2)-N-. was determined to be (1.0 +/- 0.2) x 10(10) M-1 s(-1). This reaction generates almost equal amounts of ONOOH and NO3- + H+. The overall rate constant of the reaction of (NO2)-N-. with O.-was determined to be (3-4) x 10(9) M-1 s(-1). From published thermodynamic data the equilibrium constant of homolysis of ONOO- into (NO2)-N-. and O.- is calculated to be (5.9 +/- 2.9) x 10(-16) M, and hence the calculated rate constant of homolysis of ONOO- into (NO2)-N-. and 0'- is (0.9 - 3.5) x 10(-6) s(-1). The rate constants for ONOO- decomposition at pH 13 and 14 (25 degrees C) were determined to be 1.3 x 10(-5) and 1.1 x 10(-5) s(-1), respectively, and the yield of NO2- in this process was found to be ca. 50%. On the assumption that ca. 1/2 of ONOO- decomposes via homolysis into (NO2)-N-. and O.-, a limiting rate constant for the decomposition of ONOO- can be predicted at sufficiently high pH, k(d) = (0.36 - 1.4) x 10(-5) s(-1) which is four times as high as the rate constant of homolysis into (NO2)-N-. and O.-. Both results are in excellent agreement with the homolysis model. The activation parameters for the decomposition of ONOO- at pH 14 were determined to be A = 8 x 10(10) s(-1) and E-a = 21.7 kcal/mol. The relatively low A value suggests a high degree of solvent organization in the transition state. The mechanisms of homolysis of ONOOH and ONOO- are compared and discussed in detail.