A search for external vibrations of hydrocarbons physisorbed on Cu(100) has been carried out with high-resolution inelastic He-atom scattering. For the saturated hydrocarbons n-hexane, cyclohexane, n-octane, and n-decane, the frustrated translation normal to the surface could be detected with energies of 6.8 meV, 7.3 meV, 7.0 meV, and 7.0 meV, respectively. A very similar value of 7.3 meV for the same vibration is observed for the unsaturated hydrocarbon benzene. No vibrational modes corresponding to frustrated translations parallel to the surface could be detected.