High-resolution electron energy loss spectroscopy (HREELS) has been used to investigate the inelastic electron scattering processes involved in the excitation of the normal modes of vibration of atomic hydrogen adsorbed on Si(100) and Ge(100), Detailed measurements as a function of incident electron beam energy allow a clear distinction to be made between dipole and nondipole excitation processes. The bending and scissor modes of adsorbed hydrogen are excited predominantly by dipole scattering, but the stretching modes show characteristic resonances between 3 and 6 eV. The mechanism for these scattering processes is discussed.