Products and kinetics of the thermal. decomposition of ethylsilane, under static system conditions, are reported. Both neat and propylene-inhibited reactions follow silylene chain mechanisms. Modeling fits to product yield and reactant loss data establish approximate values for the rate constants of the ethylsilylene decomposition to ethylene and silylene (k(2) approximate to 2 x 10(6) s(-1)) and the ethylsilylene isomerization to vinylsilane (k(8) approximate to 5 x 10(2) s(-1)), both at 723 K. RRKM and thermochemical analyses of these results yield values for the high-pressure activation energy of the ethylsilylene decomposition (EtSiH (2)--> C2H4 + SiH2) of E(2) = 25.0 kcal/mol and a heat of formation for ethylsilylene of Delta H-f(EtSiH) = 47.7 +/- 2.5 kcal/mol. An enthalpy group additivity for the methylene group in organosilanes and organosilylenes of Delta H-f[C-(H-2)(C)(Si)] = -1.3 +/- 2 kcal/mol is deduced as well as the activation energy of the isomerization of ethylsilylene to vinylsilane (EtSiH --> CH2 = CHSiH3) of E(8) = 35.6 +/- 2 kcal/mol.