The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, as well as other high level ab initio calculations [QCISD(T) and CCSD(T)/6-311++G(3df, 2pd, and QCISD(T)/6-311+G(5d2f, 2p)] which avoid the additivity scheme of this theory, are used to estimate the heat of formation of HSO. For this purpose, we propose also, as a further improvement of the standard G2 scheme, to evaluate the residual correlation effects by using the QCISD(TQ) method, which is exact in fifth order rather than the usual QCISD(T) procedure. The heat of formation of HSO at 0 K estimated using different reactive processes is -4.2+/-1.3 kcal/mol. The most significant consequence is that the depletion of ozone by HSO is predicted to be slightly endothermic rather than exothermic, as it has been concluded in other theoretical studies. We have also found that HSO is more stable than SOH, but the energy gap between both isomers predicted by our calculations is smaller than previous reported values.