The CCSD(T) method, singles and doubles coupled-cluster theory plus a perturbational estimate of the effects of connected triple excitations, has been used in conjunction with a double polarized triple-zeta (TZ2P) basis set to determine the equilibrium structures, dipole moment, vibrational frequencies, and infrared intensities of ClNO2, cis-ClONO, and trans-ClONO. The ab initio results are compared with the available experimental data, and the CCSD(T) results are found to be in excellent agreement with the experimental values, indicating that the CCSD(T) method performs well in describing the weak Cl-N and Cl-O single bonds. The equilibrium structures of cis- and trans-ClONO are shown to possess normal Cl-O, O-N, and N=O bond distances, in disagreement with a previous study in which the structures were determined based on a normal coordinate analysis using the experimental frequencies. It is shown that incorrect structures were obtained due to a mislabeling of the vibrational modes. A vibrational band previously assigned to an OClNO species is shown to be actually due to trans-ClONO. The relative energies of the various species are investigated by computing CCSD(T) energies using atomic natural orbital (ANO) basis sets of spdfg quality. ClNO2 is found to be more stable than cis-ClONO by 10.7 +/- 1.0 kcal/mol while trans-ClONO is 3.1 +/- 0.8 kcal/mol higher in energy than cis-ClONO. The heats of formation of cis-ClONO and trans-ClONO are predicted (15.4 +/- 1.5 and 18.0 +/- 1.5 kcal/mol, respectively; 0 K) using isodesmic reactions involving H2O, HOCl, and cis- and trans-HONO.