The mass spectra of the series of compounds Me 3MM′Me 3 and Me 4M are reported and analysed, where M and Mt́ are C, Si, Ge, Sn, and Pb. Ionisation potentials of the parent molecular ions and appearance potentials of Me 3M + and Me 3M′ + fragment ions have been measured. Using known or estimated values for the gas phase standard enthalpies [Δ H o f (g)] of formation of Me −, Me 3C −, Me 3C +, Me 4M, Me 6C 2, Me 6Si 2, and Me 6Sn 2 yields an overdetermined set of 31 simultaneous equations for the Δ H o f (g) of 27 molecules, radicals, and ions. A least squares analysis thus affords the Δ H o f (g) data for all the species Me 3MM′Me 3, Me 4M, Me 3M −, and Me 3M +. From these data, values are derived for bond ( D) and mean bond ( D ) dissociation energies: D(Me 3MMe), D(Me 3MM′Me 3), and D (Me 4M), as well as meant hermochemical bond energy terms ( E ) for MeM and MMt́ bonds in each of these species. Data (in kcal/mole) include ( i). E (MeM) in Me 4M (value for M in parentheses): 82.1(c), 68.5(Si), 59.9(Ge), 48.1(Sn), and 32.9(Pb); ( ii). E(MM′) in Me 6M 2 : 78.9(c), 68.0(Si), 59.3(Ge), 38.3(Sn), and 33.3(Pb); ( iii). E(MC) in Me 3MCMe 3 : 66.6(Ge), 53.0(Sn), and 41.6(Pb); ( iv). E(MSi) in Me 3MSiMe 3 : 63.3(Ge) and 56.1(Sn), and ( v). E(GeSn) in Me 3GeSnMe 3 : 53.7. Trends are discussed.