The decomposition of triethylgallium (TEGa-d(0)) and perdeuterated triethylgallium (TEGa-d(15)) on Ga-rich GaAs (100) was studied using temperature programmed static secondary ion mass spectrometry (SSIMS) and temperature programmed desorption (TPD). Isothermal and temperature programmed SSIMS spectra of the TEGa-dosed surface feature three dominant hydrocarbon ions, CH3+, C2H3+, and C2H5+, whose behavior indicates the presence of two distinct ethyl ligands. The TEGa-d(15) results were equivalent to the TEGa-d(0) results, with the ion masses shifted by the D and H mass differences. Both CH3+ and C2H3+ SSIMS signals exhibit the same temperature dependence, staying rather constant between 140-530 K and then decreasing rapidly between 530 and 650 K. The CH3+ and C2H3+ signals disappear over the same temperature range in which C2H4 forms and desorbs from the surface in TPD; these SSIMS signals are attributed to the surface ethyl ligands that are responsible for C2H4 formation. Ethyl ligands reacting to ethylene are likely bound to Ga atom lattice sites. The C2H3+ signal is well established at 140 K, suggesting that TEGa is dissociated at temperatures as low as 140 K. The C2H5+ SSIMS signal decays in parallel with the diethylgallium ion SSIMS signal, and it then disappears in parallel with the CH3+ and C2H3+ signals; the C2H5+ SSIMS signal is associated with both ethyl ligands bound to adsorbed triethylgallium and/or diethylgallium, and with ethyl ligands bound to lattice sites.