Abstract The phonon density of states of Rb 3C 60 has been measured above and below the superconducting phase transition temperature of 29 K and in the energy transfer range 1 <Δ E<300 meV at an energy resolution Δ E/ E = 2%. Marked changes in the molecular vibrational spectrum of C 60 are observed upon intercalation with rubidium. In particular the H g modes at 54 meV and the T 1u mode at 66 meV are strongly quenched and other modes are broadened and shifted. Distinct changes in the H g, T 1u modes near 180 meV and the H g modes near 135 meV are observed on passing through the superconducting transition temperature region. These changes strongly suggest the involvement of molecular motions in the superconductivity of these materials.