The energies of six stationary points on the OH+CO --> HOCO --> H+CO2 potential energy surface have been calculated using the G3 and CBS-QB3 methods. An analysis combining ab initio and experimental enthalpies yielded Delta H-f(298 K) (trans-HOCO)=-42.9 +/- 1.5 kcal mol(-1) (-43.8 +/- 1.4 kcal mol(-1)) at the G3(CBS-QB3) level of theory. These results confirm the revised HOCO heat of formation derived from photoionization spectroscopy and suggest that the HOCO potential well is 8.8 kcal mol(-1) shallower than previously thought. We discuss the implications of these results for accurate Rice-Ramsperger-Kassel-Marcus modeling or quantum mechanical scattering calculations of the OH+CO reaction.