The intensity of the electron-energy loss from vibrational modes of hydrocarbons adsorbed on aluminum and indium was measured as a function of the primary-beam energy in the range 6 13 eV. We have found a resonance in the vibrational-excitation cross section at an energy about 0.5 eV below the energy of the substrate-metal surface plasmon. A model is proposed, in which resonant excitation of a surface plasmon, followed by trapping of the primary electron in an image-dipole induced surface state, takes place in the intermediate stage of the total excitation process. The excitation of the vibrational mode in this intermediate stage occurs either by the electron or by the excited surface plasmon. The results of our model calculation agree quite well with experiment.