On the basis of the Gaussian-2 theory of molecular energies, supplemented with empirical data, we have determined the ideal gas heat of formation of HOBr, Delta H(f)(d)egrees(300), to be -14.2 +/- 1.6 kcal mol(-1). This is in contrast to the widely used estimate of -19 kcal mol(-1) and the recently suggested value of -19 kcal mol(-1). Thermodynamic data for HOBr are derived in the 0-500 K range and used to compute reaction energies of importance to atmospheric chemistry.