The reactions of dimethylzinc (DMZ) and diethylzinc (DEZ) on the As-rich c(4X4) reconstruction of GaAs(100) were studied using temperature programed desorption. Both DMZ and DEZ dissociatively adsorbed on the c(4x4) surface to form adsorbed Zn atoms and alkyl groups. The primary reaction pathway for surface methyl groups formed via dissociative adsorption of DMZ was desorption as methyl radicals between 500 and 720 K. A small fraction of the methyl groups also underwent dehydrogenation to form adsorbed hydrogen atoms and deposit carbon on the surface. In contrast, for surface ethyl groups formed from DEZ, the primary reaction pathway was beta-hydride elimination to produce ethylene. Comparison of the results obtained in this study with those reported previously for the Ga-rich (4x2) reconstruction of GaAs(100) indicates that surface Ga atoms enhance the dehydrogenation activity of GaAs(100) surfaces.