Hydrogen adsorption on the c(4X4), (2X4), (2X6), and (4X2) reconstructions of GaAs (001) have been characterized by internal-reflection infrared spectroscopy. The infrared spectra contain up to 15 bands due to the stretching vibrations of arsenic hydrides (2150-1950 cm(-1)), terminal gallium hydrides (1950-1800 cm(-1)), and bridging gallium hydrides (1800-950 cm(-1)). These features arise from hydrogen adsorption on arsenic and gallium dimers, and second-layer arsenic and gallium atoms. The large number of peaks observed indicates that the surface atoms exist in a variety of different chemical environments.