Abstract The time-of-arrival distributions of trimethylgallium (TMG) scattered from variously relaxed and reconstructed GaAs(100), -(110), and -(111)B surfaces, as well as sticking coefficients of TMG on these surfaces, have been measured using pulsed molecular beams. It is concluded that TMG scattering is well described within the framework of precursor-mediated chemisorption, which is determined by the depth of the precursor state and the barrier height to chemisorption. The surface-structure dependence of the depth of the precursor state, estimated from the temperature-dependence of the surface residence time during scattering, is explained by taking into account the charge distribution in the relaxed or reconstructed surface structure. The barrier height to chemisorption is interpreted as the stability of the relaxed or reconstructed surface structure. The higher suppression of TMG decomposition on the (2 × 2)-reconstructed GaAs(111)B surface, utilized in the lateral growth of GaAs, is considered to be due to the absence of localized charge at the topmost As atoms. We propose that the topmost As placed on an As-terminated surface hinders the trapping and sticking of TMG.