The gas phase electronic structures of CM(3)C(O)ONO and CM(3)C(O)ONO(2) (M=H, Cl, F) are studied by photoelectron spectroscopy (PES) combined with the outer valence Green's function (OVGF) calculations at 6-311+G(d, p) basis sets. The highest occupied molecular orbital (HOMO) for each compound is the carbonyl oxygen lone pair (n(O)), the ionizations of these orbitals are associated with the vibrational frequency about 1750 and 1820 cm(-1) reflected on the first band, respectively, for acyl nitrites and nitrate. Comparing with the calculated energies, it can be concluded that the syn conformers with Cs overall symmetry, a planar CC(O)ONO skeleton in nitrites, and a planar CC(O)ON skeleton in nitrates, respectively, are the most stable in the gas phase.