In this work, we studied the reactions of alkyl peroxyl radicals with (NO2)-N-. and (NO)-N-. using the pulse radiolysis technique. The rate constants for the reaction of (NO2)-N-. with (CH3)(2)C(OH)CH2OO., CH3OO. and c-C5H9OO. vary between 7 x 10(8) and 1.5 x 10(9) M-1 s(-1). The reaction produces relatively long-lived alkyl peroxynitrates, which are in equilibrium with the parent radicals and have no appreciable absorption above 270 nm. It is also shown that (NO)-N-. adds rapidly to (CH3)(2)C(OH)CH2OO. and CH3OO. to form alkyl peroxynitrites. The rate constants for these reactions were determined to be 2.8 x 10(9) and 3.5 x 10(9) M-1 s(-1), respectively. However, in contrast to alkyl peroxynitrates, alkyl peroxynitrites do not accumulate. Rather, they decompose rapidly via homolysis along the relatively weak O-O bond, initially forming a geminate pair. Most of this pair collapses in the cage to form an alkyl nitrate, RONO2, and about 14% diffuses out as free alkoxyl and (NO2)-N-. radicals. A thermokinetic analysis predicts the half-life of CH3OONO in water to be less than 1 mus, an estimate that agrees well with previous experimental findings of ours for other alkyl peroxynitrites. A comparison of aqueous and gaseous thermochemistry of alkyl peroxynitrates reveals that alkyl peroxyl radicals and the corresponding alkyl peroxynitrates are similarly solvated by water.