We report here results dealing with the effects of H2S adsorption on crystallographic and electronic surface properties of GaAs {100} layers grown in situ by molecular beam epitaxy (MBE). The adsorption was studied by AES, LEED and ELS. At room temperature, the adsorption is found to be very dependent on the electron irradiation, whereas this effect vanishes at 700 K. This behavior indicates that the mechanism of adsorption is temperature dependent: H2S is adsorbed mainly in molecular form at low temperature (300 K) and in dissociated form at higher temperature (700 K). Adsorption of the molecular form leads to a disordered layer, whereas the dissociative adsorption process induces a (2×1) reconstruction of the GaAs surface near the saturation coverage. This structural reordering is also associated with changes observed in the loss spectra. Moreover, Auger measurements indicate that it is characterized by significant changes in arsenic and gallium lines: it is most likely that the (2×1) structure corresponds to an important modification of the sulfur bonding at the surface. To tentatively investigate the influence of the nature of the semiconductor surface in the determination of the metal/GaAs interface properties, electrical characteristics of in situ Al contacts on H2S saturated surfaces were investigated and compared to those obtained on virgin surfaces. Contacts on clean MBE layers exhibit nearly ideal Schottky behavior (barrier height 0.76 eV, ideality factor 1.01) both as deposited (300 K) and after annealing up to 800 K. On the contrary, room temperature contacts on H2S saturated surfaces were found to be degenerate.