The velocity distribution of particles scattered from surfaces can provide information about the dynamics and energetics of the interaction process. In a series of preliminary measurements on clean, well−characterized GaAs (111) we have measured the velocity distributions of both scattered and evaporated As2 and As4, with substrate temperatures ranging from 25° to 700°C. The results indicate that the molecular species As2 and As4 both scatter and evaporate from the surface with velocity distributions ’’cooler’’ than that characteristic of the substrate temperature. On the other hand, the Ga velocity distributions are well described by a Maxwell−Boltzmann distribution at the substrate temperature. In addition, the experiments provide an unambiguous identification of species leaving a surface, establishing in this case that As2 is the only detectable arsenic species evaporating from GaAs at 500°C.