The standard singles and doubles coupled-cluster method including perturbational treatment of connected triple excitations, CCSD(T), and density functional methods, DFT, using a large, uncontracted, atomic natural orbital (ANO) basis set were employed for calculating the enthalpies of formation of some first-row atoms (H, N, O, F) as well as diatomic and triatomic molecules formed with them. Molecular enthalpies of formation at 298.15 K were obtained from enthalpies of reaction of the atoms, homonuclear diatomic molecules, and isodesmic reactions. It is shown that the errors are the minimum when isodesmic reactions are used. However, contrary to accepted belief, CCSD(T) gives a larger deviation from experiment than DFT in the latter case. DFT exhibits similar accuracy when using a very small basis set (6-31G*) to that with the extended basis set, while the errors obtained with CCSD(T) are much larger. It is concluded that DFT is a more accurate and convenient computational tool than CCSD(T) for the thermochemical study of species containing the F-O bond.