The full vibrational spectra of alpha and beta crystalline phases of syndiotactic polystyrene (sPS), that is, phases presenting the trans-planar conformation, have been experimentally determined and compared with that calculated at the B3LYP/6-31G(d,p) level of theory for an infinite trans-planar chain. The normal vibrational analysis of most representative modes of the periodic model allowed us to give a general description of each one, which was further confirmed by the direct inspection of mode animations. An assignment of the different modes was performed in terms of frequency, relative intensity, and direction of the transition-moment vector of the observed IR peaks as well as Raman vibrational frequencies.