To determine the heat of formation of boron atom, a fundamental parameter in gas-phase thermochemistry, from a thermochemical cycle involving BS, the total atomization energy (TAE(0), Sigma D-0) of this molecule was accurately determined ab initio in an exhaustive convergence study using coupled cluster [CCSD(T)] methods. Basis sets up to [8s7p6d5f4g3h2i] quality have been considered, and an extrapolation for further basis set incompleteness was applied. Inner-shell correlation, anharmonicity in the zero-point energy, and atomic spinorbit splitting have all been taken into account. Our best computed TAE(0) for BF3, 462.6 +/-0.3 kcal/mol, leads to a best heat of formation for gaseous boron at 0 K of 136.0 +/- 0.4 kcal/mol, in excellent agreement with an experimental determination of 136.2 +/- 0.2 kcal/mol and definitively confirming recent suggestions that the established reference value, 132.7 +/- 3.0 kcal/mol, should be revised. This revision will affect most known gas-phase thermochemical data for boron compounds. As a byproduct, we obtain a dissociation energy D-0 of 180.13 +/- 0.2 kcal/mol for the BF diatomic, in perfect agreement with experiment but with a much smaller uncertainty.