The heats of formation of ClCO, Cl2CO, BrCO, and Br2CO have been calculated at high levels of ab initio molecular orbital theory. Geometries and frequencies for the two closed shell molecules were calculated at the MP2 level. The geometries of the two radicals were calculated at the level of coupled cluster theory with single and double excitations including a perturbative treatment of the connected triple excitations (CCSD(T)) with the augmented correlation consistent triple-xi basis set (aug-cc-pVTZ). Extrapolation of the total energies (aug-double, aug-triple, aug-quadruple) to the complete 1-particle basis set limit was performed to further reduce the basis set truncation error. Additional improvements in the atomization energy were achieved by applying corrections for core/valence correlation, scalar relativistic effects, and spin-orbit coupling. Zero point energies were based on MP2 calculated values. Using restricted open shell treatments for the atoms, we find the following heats of formation (kcal/mol) at 0 K: Delta H-f[C(O)Cl-2] = -51.6 +/- 0.5 (calc.) vs -52.2 +/- 0.8 (expt.); Delta H-f(ClCO) -4.6 +/- 0.5 (calc.) vs -5.6 +/- 0.7 (expt.); and Delta H-f[C(O)Br-2] = -22.7 +/- 1.0 (calc.) vs -23.3 +/- 0.1 (expt.), and Delta H-f[BrCO] = 0.7 +/-1.5 (calc). The radical BrCO is predicted to be only weakly bound with a long quasi-linear Br-C bond length of 3.09 Angstrom, and a Br-CO binding energy (Delta E-elec) of only 1.6 kcal/mol calculated at the CCSD(T) estimated basis set limit. Inclusion of both molecular and atomic spin-orbit coupling effects reduces this to just 0.3 kcal/mol.