Trimethylgallium (TMGa) chemisorption on the gallium‐rich ‘‘(4×6)’’ and (1×6) GaAs(100) surfaces has been studied with temperature programmed desorption (TPD) and static secondary ion mass spectroscopy (SSIMS). Low coverages of TMGa irreversibly dissociate on GaAs(100) and methyl radicals are the main gas‐phase product observed around 700 K during TPD. At higher TMGa coverages some recombinative desorption of molecular TMGa occurs from 250–650 K. On the Ga‐rich surface reconstructions the desorbing methyl radicals are kinetically indistinguishable, strongly suggesting that they evolve from a single type of binding site. We believe this binding site corresponds to surface MMGa, which is formed by stepwise methyl group transfer from chemisorbed TMGa to neighboring surface gallium atoms. The presence of MMGa is supported by SSIMS results at ∼650 K. Temperature‐programmed SSIMS was used to follow the dissociation of adsorbed TMGa from 120 to 700 K.