The kinetics and mechanism of arsine adsorption on the (4 x 2) surface of gallium arsenide (001) has been studied by scanning tunneling microscopy, infrared spectroscopy, and ab initio quantum chemistry calculations. Arsine forms a dative bond to a gallium dimer, Then, this species either desorbs from the surface or decomposes to an AsH2 or AsH fragment with hydrogen transfer to an arsenic site. Finally, desorption of hydrogen leaves arsenic dimers on the surface. The energy barriers for arsine desorption and dissociation into AsH2 are estimated to be 9.3 and 16.5 kcal/mol, respectively. Gallium hydride is not produced upon dissociation of AsH3 because this process is not energetically favorable.