Hildenbrand and co-workers have shown recently that the vapor above solid ammonium nitrate includes molecules of NH4NO3, not only NH3 and HNO3 as previously believed. Their measurements led to thermochemical values that imply an enthalpy change of D-298 = 98 +/- 9 kJ mol(-1) for the gas-phase dissociation of ammonium nitrate into NH3 and HNO3. Using updated spectroscopic information for the partition function leads to the revised value of D-298 = 78 +/- 21 kJ mol(-1) (accompanying paper in this journal, Hildenbrand, D. L.; Lau, K. H.; Chandra, D. J. Phys. Chem. B 2010, DOI: 10.1021/jp105773q). In contrast, high-level ab initio calculations, detailed in the present report, predict a dissociation enthalpy half as large as the original result, 50 +/- 3 kJ mol(-1). These are frozen-core CCSD(T) calculations extrapolated to the limiting basis set aug-cc-pV infinity Z using an anharmonic vibrational partition function and a variational treatment of the NH3 rotor. The corresponding enthalpy of formation is Delta H-f(298)o(NH4NO3,g) = 230.6 +/- 3 kJ mol(-1). The origin of the disagreement with experiment remains unexplained.