The methyl cation affinities of the rare gases have been calculated at 0 and 298 K by using coupled cluster theory including noniterative, quasiperturbative triple excitations with the new correlation-consistent basis sets for Xe up through aug-cc-pV5Z in some cases. To achieve near chemical accuracy (± 1 kcal/mol) in the thermodynamic properties, we add to the estimated complete basis set valence binding energies, based on frozen core coupled cluster theory energies, two corrections: (1) a core/valence correction and (2) a scalar relativistic correction. Vibrational zero-point energies were computed at the coupled cluster level of theory at the CCSD(T)/aug-cc-pVDZ level. The calculated rare gas methyl cation affinities (MCA in kcal/mol) at 298 K are the following: MCA(He) = 1.7, MCA(Ne) = 2.5, MCA(Ar) = 16.9, MCA(Kr) = 25.5, and MCA(Xe) = 36.6. Because of the importance of the MCA(N-2) in the experimental measurements of the MCA scale, we calculated a number of quantities associated with CH3N2+ and CH2N2. The calculated values for diazomethane at 298 K are: &UDelta; H-f(CH2N2) = 65.3 kcal/mol, PA(CH2N2) = 211.9 kcal/mol, and MCA(N-2) = 43.2 kcal/mol.