Nitrogen dioxide (NO2.), pruduced I;y photodecomposition (lambda > 200 nm) of polycrystalline sodium nitrite (Na+NO2-), polycrystalline energetic oxidizers, ammonium nitrate (NH4+NO3-), and ammonium dinitramide [NH4+N(NO2)(2)(-)], and the polycrystalline cyclic nitramine cyclotrimethylenetrinitramine (RDX), was studied by electron spin resonance (ESR) spin trapping at room temperature (300 K) and by matrix isolation ESR spectroscopy at 77 K. In spin trapping experiments, the aci anion of nitromethane (CH2=NO2-) was used to spin trap NO2. produced by photodecomposition of these energetic compounds in basic (pH > 13) aqueous solutions at room temperature. The resulting NO2. adduct radical is the dinitromethyl anion radical (DNM(.)) ((NO2)-N---CH=NO2.-) which gives a 10-line ESR spectrum. For equimolar starting concentrations of each compound, the ESR signal;intensities of the DNM(.) spectrum varied according to NaNO2 > NH4N(NO2)(2) > NH4NO3 > RDX. From kinetics of DNM(.) formation, the suggested dominant mechanisms of NO2. formation are NO2- + OH. - NO2. + OH- for compounds NaNO2 and RDX and NO3- --> NO2. + O- for NH4NO3 and NH4N(NO2)(2). Two Other cyclic-nitramines, cyclotetramethylenetetranitramine (HMX) and hexanitrohexaazaisowurtzitane (HNIW), were confirmed to give weak DNM(.) adduct ESR signals. In matrix isolation ESR experiments at 77 K, the rate of formation of NO2. is zero-order for NaNO2 and NH4NO3 but first-order for NH4N(NO2)(2) under the photolysis conditions of this experiment, suggesting that, in the solid phase, NO2. is formed by different mechanisms in the two ammonium salts.