Singles and doubles coupled-cluster (CCSD) theory and the CCSD(T) method, CCSD plus a perturbational estimate of the effects of connected triple excitations, have been used to determine the equilibrium structures, dipole moment, vibrational frequencies, and infrared intensities of the HNO, FNO, and ClNO species. A double polarized triple-zeta (TZ2P) quality basis set was used. The CCSD (T) results are found to be in very good agreement with experiment, indicating that the CCSD(T) method performs well in describing the weak X-NO single bond. The experimental heats of formation of the FNO, and ClNO species are examined for internal consistency with experimental data for FNO2 and ClNO2, by using an isodesmic reaction. It is concluded that the experimental data exhibit a small inconsistency. An approach, involving in addition the use of homodesmic reactions, by which a more consistent set of thermochemical data may be obtained is proposed and discussed.