The decomposition of trimethylgallium (TMGa) on the gallium‐rich (4×6) and (1×6) GaAs (100) surface was studied with temperature programmed desorption, Auger electron spectroscopy, and low‐energy electron diffraction. TMGa was found to dissociatively chemisorb on the gallium‐rich surfaces, apparently at the gallium vacancies that exist on these surfaces. We have unambiguously identified methyl radicals desorbing from the surface with the maximum rate at ∼440 °C following a saturation TMGa exposure. Since TMGa was shown to decompose on the clean, gallium‐rich GaAs (100) surfaces, the self‐limiting deposition of gallium during atomic layer epitaxy must be due to the presence of surface methyl groups which inhibit further TMGa dissociative chemisorption.