High-level ab initio calculations and variable-temperature proton-transfer equilibrium constant measurements have been used to obtain new thermochemical data for protonated halogenomethanes (CH(3)XH(+), X = F, Cl, Pr, and I) and protonated diazomethane (CH3NN+). Proton affinities of CH(3)X and CH2NN and methyl cation affinities of HX and N-2 have been derived. The theoretical and experimental results are in good agreement with one another but in several cases are in conflict with currently accepted experimental proton and methyl cation affinities. Experimental and theoretical methyl cation affinities are presented for a variety of molecules, leading to the proposal of a new methyl cation affinity scale.