Gas-phase methyl cation affinities (MCAs) for rare gases Ne, Kr, and Xe were measured with a pulsed electron-beam high-pressure mass spectrometer. The MCAs for Ne and Kr were determined to be 1.2 +/- 0.3 and 19.8 +/- 2.0 kcal/mol, respectively, by the observation of the clustering reaction, CH3+ + Rg = CH3+(Rg) (Rg = Ne and Kr). The MCA of Xe was measured to be 2.0 +/- 0.6 kcal/mol larger than that of N-2 by the observation of the substitution reaction CH3+(N-2) + Xe = CH3+(Xe) + N-2. Based on the MCA of N-2 of 44.1 kcal/mol proposed by McMahon et al., the MCA of Xe is determined to be 46.1 +/- 0.6 kcal/mol. Molecular orbital calculations at six different levels consistently gave almost identical MCA values for each of the rare gases. At QCISD(T)(full)/6-311++G(2df,p)//B3LYP/6-311++G(d,p), the calculated values (all in kcal/mol) are as follows: He, 0.6; Ne, 2.2; Ar, 15.9; Kr, 24.1; and N2, 43.2. For Xe at B3LYP/DZVP//B3LYP/DZVP, the calculated MCA is 39.0 kcal/mol. The ethyl cation affinities of Ar, Kr, and Xe were also measured. They are similar to1.7, 3.2 +/- 0.3, and 6.8 +/- 0.3, respectively. The stabilities Of C2H5+(Rg) and C2H5+(N-2) were discussed in terms of nonclassical (bridge) and classical (open) structures Of C2H5+.