One of the possible key products in the oxidation and transport of atmospheric mercury is mercury monoxide, HgO. In the present work, large-scale multireference configuration interaction, as well as coupled cluster, wave functions have been used in conjunction with a series of correlation consistent basis sets to calculate the near-equilibrium potential energy functions of the first two electronic states of HgO. In the absence of spin-orbit coupling, the lowest (1)Sigma(+) and (3)Pi states are nearly isoenergetic with a binding energy with respect to ground-state atoms of only 2-3 kcal/mol at the estimated complete basis set limit. After spin-orbit coupling effects are accurately included, the resulting X0(+) state has a D-0 of just 4.0 kcal/mol, leading to a DeltaH(f)(0 K) of 70.4 kcal/mol. These values are in stark contrast to the currently accepted experimental values. The implications of these results to atmospheric mercury chemistry are briefly discussed.