The nearly dispersionless, so-called ‘optical’ vibrational modes observed by inelasticneutron scattering from icosahedral Al–Pd–Mn and Zn–Mg–Y quasicrystals are foundto correspond well to modes of a continuum elastic sphere that has the samediameter as the corresponding icosahedral basic units of the quasicrystal. Whenthe sphere is considered as free, most of the experimentally found modes can beaccounted for, in both systems. Taking into account the mechanical connectionbetween the clusters and the remainder of the quasicrystal allows a completeassignment of all optical modes in the case of Al–Pd–Mn. This approach providessupport to the relevance of clusters in the vibrational properties of quasicrystals.