The halomethylidynes CH, CF, CCl, CBr, and CI have been investigated using GAUSSIAN-2 theory and calculations up to QCISD(T)/6-311+G(3df, 2p). At the latter level of theory bond lengths, harmonic frequencies, and anharmonicity corrections have been derived and compare well with experimental measurements where these are available. GAUSSIAN-2 and QCISD(T)/6-311+G(3df, 2p) energies were employed in isodesmic reactions to obtain the following recommended enthalpies of formation (Delta(f)H(298)) in kJ mol(-1):CF, 248.6; CCl, 436.9; CBr, 497.1; CI, 557.6. Estimated error limits are +/-6 kJ mol(-1).