The room temperature deposited α-P/GaAs(100) interfaces have been studied by XPS, UPS, and LEED. The results show that P is adsorbed as clusters on the surface of GaAs at the initial stage of the interface formation, α-P film is formed as the deposition amount is further increased. The valence structures of the films so obtained are similar to those of plasma deposited α-P:H films. There are about one monolayer of P atoms bonded to Ga atoms of the substrate at the interface, α-P overlayer results in 0.2 eV lowering of GaAs surface barrier.