The reactions of NO. with O2.- and with HO2. were studied using the pulse radiolysis technique under pseudo first order conditions where ([O2.-]o + [HO2.]o) > [NO.]o at pH 3.3-10.0. The rate constant of the reaction of NO. with O2.- was determined both by monitoring the decay of O2.- at 250 nm and the formation of ONOO- at 302 nm to be (4.3 +/- 0.5) x 10(9) M-1s-1, independent of ionic strength and pH in the range of 6.1-10.0. The rate constant of the reaction of NO. with HO2.- was determined by following the decay of HO2. at 250 nm to be (3.2 +/- 0.3) x 10(9) M-1s-1 at pH 3.3.