Several isomers of nitrobenzene (1) and nitrobenzene cation-radical (1(+.)) were identified as stable structures and their gas-phase chemistry was investigated by the methods of tandem mass spectrometry and combined ab initio and density functional theory calculations. The relative energies of [C-6,H-5,N,O-2] neutral molecules followed the order o-nitrosophenol (5, most stable) < p-nitrosophenol (7) < o-benzoquinonenitrone (10) < p-benzoquinoneoxime (19) < o-benzoquinoneoxime (11) < m-nitrosophenol (6) < nitrobenzene (1) < phenyl nitrite (8) < benzo[d]-1,3-dioxa-2-azolidine (13) much less than triplet didehydrocyclohexa-2,4-dien-1-yliden-1-nitronic acids, para (((3)A") 4) approximate to meta (((3)A") anti3) < ortho (((3)A") syn-2) < singlet didehydrocyclohexa-2,4-dien-1-yliden-1-nitronic acids, meta (('A) syn-3) < ortho (((1)A) syn-2) < para (('A) 4) (least stable). Molecules 1, 5, 7, and 8, were generated in the gas phase by femtosecond collisional electron transfer and their dissociations were investigated by neutralization-reionization mass spectrometry. The relative energies of [C-6,H-5,N,O-2](+.) cation radicals followed the order 7(+.)(most stable) < 5(+.) < 6(+.) < anti-11(+.) < 8(+.) < 10(+.) < 13(+.) < benzo[al-l,2-dioxa-3-azolidine (12(+.)) < 1(+.) < 4(+.) < syn-3(+.) < syn-(2.) (least stable).