We have investigated the mechanism of H2S adsorption and the evolution of a sulfur passivation layer on the GaAs(001)-(4x2) surface using high-resolution electron energy loss spectroscopy, temperature-programmed desorption, Auger electron spectroscopy, and low-energy electron diffraction. The initial H2S adsorption occurs dissociatively to form HS and H with the hydrogen selectively bonded to As at 100 K. Upon annealing to 700 K, vacant Ga sites, which are available for further dissociation of H2S, are monitored by postexposure of atomic hydrogen. The repeated cycles of H2S exposure at 100 K and thermal annealing yielded a sulfur-saturated layer on which only sulfur is available for posthydrogenation. We have also examined the passivation performance of the H2S-treated surface when exposed to water at 100 K.