The adsorption of H2S on GaAs(100) surfaces has been studied using high-resolution electron energy-loss spectroscopy, temperature-programed desorption, Auger electron spectroscopy, and low-energy electron diffraction. Dissociative adsorption of H2S to HS and H via a molecularly adsorbed precursor has been observed on both GaAs(100)-(2x4) and GaAs(100)-(4X2) surfaces. Upon exposure of H2S at 100 K, dissociative adsorption occurs prior to adsorption of the more weakly bound, molecularly adsorbed H2S. The dissociated H2S desorbs as AsH3 and S-2 at 600 K from the As-rich GaAs(100)-(2X4) surface, suggesting that As-SH bonds were formed from the dissociative adsorption of H2S, which decomposes at 600 K. On Ga-rich GaAs(100)-(4x2), desorption of AsH3 is observed over a wide range of temperatures from 200 to 550 K, although to a lesser extent than from the As-rich surface, and H-2 desorption from Ga sites is observed at 500 K. Desorption of S-2 was not observed on GaAs(100)-(4X2), suggesting that Ga-SH bonds were formed from the dissociative adsorption of H2S on GaAs(100)-(4x2).