Hydrogen adsorption on two-dimensional (2D) ErSi 2 /Si(111) as well as on a 300 thick ErSi 1 . 7 (0001) film on Si(111) has been studied by HREELS. Both surfaces exhibit the characteristic monohydride Si-H bending and stretching modes confirming the structural model of a silicide terminated by a buckled silicon plane similar to the ideal Si(111) termination. Nevertheless when compared to the Si(111)-δ(7 7)H or Si(001)-(2 1)H surfaces, these modes are found to be shifted towards lower energies: approximately 25 cm - 1 for the 2D silicide and 65 cm - 1 for the thick silicide. This provides an example of a marked change in the Si-H surface vibration frequencies induced by a modification in the third atomic layer below the surface i.e. in the third coordination shell of the adsorbate. The shift is interpreted as a weaker Si-H force constant on the silicide apparently related to a small reduction in bond order because of electron donation from the Er layer underneath.