High resoln. electron energy loss spectroscopy (HREELS) has been used to locate a neg. ion resonance at about 2.1eV electron beam energy in benzene chemisorbed on Pt(111). We det. the predominant excitation mechanisms for several vibrational modes of the adsorbate. A group theor. anal. of the resonantly excited modes is used to deduce the symmetry of the adsorbate complex. Emphasis is laid on the influence of the vibronic coupling to neighboring resonances. We come to the conclusion that the symmetry of the adsorbate complex should be either C3v(σv) or Cs(σv).