A new application of electron paramagnetic resonance (EPR) spin trapping is described using the nitromethane aci anion (CH2=NO2-) spin trapping technique for quantitative measurement of nitrogen dioxide radicals ((NO2)-N-.) and nitrite ions (NO2-) in strong alkaline aqueous solution. The relative concentration of a spin adduct radical, dinitromethyl dianion radical [-O2N-CH=NO2.-] (I), produced by ultraviolet photolysis, is shown to be proportional to the concentration of NO2- dissolved in alkaline solution. This method is applied to estimate the (NO2)-N-. concentrations produced by decomposition of two energetic nitramine compounds : cyclotrimethylenetrinitramine, C6H6N6O6 (RDX), and ammonium dinitramide, NH4+N(NO2)(2)(-) (ADN), by using calibration plots of the spin concentration of I versus time of photolysis. Calibrations were prepared using sodium nitrite (NaNO2) or ammonium nitrate (NH4NO3) which behave similarly to the energetic compounds with respect to formation of I.