Hydrogen adsorbed Si(100)-2 × 1 surfaces have been studied by low-energy recoil-ion spectroscopy (LE-RIS). It was found that the adsorption processes could be easily monitored by observing the hydrogen positive-recoils associated with the Ne + ion beam having the energy between 500 and 2000 eV. A detection sensitivity of the surface hydrogen was estimated to be 0.03 monolayer (ML) based on the results of elastic recoil detection analysis (ERDA). For a Si(100)-2 × 1 : H monohydride surface, hydrogen recoil ion intensity profiles showed periodic structures as a function of azimuthal angle, reflecting a surface atomic arrangement. From these periodic structures and computer simulations, a possible hydrogen adsorption site model was proposed.