The decomposition reaction mechanism of the anions generated by atmospheric pressure chemical ionization (APCI) mass spectrometry, in the negative mode, of nitroanilines and 2,4-dinitroanilines has been probed using quantum chemical calculations. The same process has been analyzed for the neutral counterparts and the simpler neutral and anionic nitrobenzenes. Our computations using density functional theory at the B3LYP/6-311++G(3df,2p) level demonstrate that the decomposition of the anion occurs in a two-stage process, involving an initial nitro-nitrite rearrangement followed by a NO-elimination leading to formation of a phenoxy radical anion derivative. The former is by far the rate-determining step. Calculations using the MP2 and CCSD(T)/6-311++G(d,p) levels also pointed out the same energy landscape. A similar mechanism has also been emphasized for the neutral counterparts. The effects of the amino and nitro groups as well as that of the negative charge on the nitro-nitrite interconversion are rather small and do not qualitatively modify the landscape of the pertinent energy surface and thereby the NO-loss mechanism.