The interaction of K with the Si(100)(2X1)-H surface has been investigated using high-resolution electron-energy-loss spectroscopy. For a low fractional K coverage (THETA(K)=0.1), a broadening of the quasielastic peak was observed. On the basis of its temperature dependence and by a comparison with electron-scattering theory, it is proposed that K adatoms form localized donor levels lying below the conduction band, which is regarded as a two-dimensional electronic system because of the downward band bending near the surface, and that the origin of the broadening is the multiple excitation of plasmons associated with electrons that are thermally excited from the donor levels into the conduction band. A narrowing of the quasielastic peak was observed with increasing THETA(K), and is attributed essentially to the displacement of hydrogen upon K adsorption. The Si-H stretching energy decreases by approximately 20 meV with increasing THETA(K), Which is attributed to the Si-H bond weakening due to charge transfer from the K adatoms. The adsorption of H atoms on the K-covered Si(100) surface is briefly discussed.