Negative hydrogen ion formation is studied by scattering protons from a cesiated tungsten (110) surface. The primary energy ranges from 50 to 400 eV. The angle of incidence is 70degree sign with respect to the surface normal. A maximum conversion efficiency H-/(H- + H0) of 67% is measured. The measurements can be described in terms of the probability model. The perturbation of the H- ion by the metal is described within first order perturbation theory. A reasonable agreement between theory and experiment is obtained.