Experimental mass-spectrometry data on thermochemistry of methide transfer reactions (CH3)3M(+) + M'(CH3)4 ↔ M(CH3)4 + (CH3)3M'(+) (M, M' = Si, Ge or Sn) and the formation energy of the [(CH3)3Si-CH3-Si(CH3)3](+) complex are used as benchmarks for DFT methods (B3LYP, BMK, M06L, and ωB97XD). G2 and G3 theory methods are also used for the prediction of thermochemical data. BMK, M06L, and ωB97XD methods give the best fit to experimental data (close to chemical accuracy) as well as to G2 and G3 results, while B3LYP demonstrates poor performance. From the first three methods M06L gives the best overall result. Structures and formation energies of intermediate "mixed" [(CH3)3M-CH3- M'(CH3)3] complexes not observed in experiment are predicted. Their structures, better described as M(CH3)4∙[M'(CH3)3](+) complexes, explain their fast decompositions.