The i.r. spectra of trans-polyalkenamers both in the amorphous and in the crystalline state have been studied. The spectra of the amorphous polymers are interpreted on the basis of the known correlations of olefinic hydrocarbons. In the spectra of crystalline polymers, bands characteristic of the crystalline phase are in closely similar positions for polymers having similar chain packings. This similarity does not occur in polymers having the same chain structure but different chain packings: thus the spectra of modification I of odd trans-polyalkenamers are analogous to those of modification IV of even trans-polyalkenamers, whereas only few similarities are observed between the spectra of modification III and those of modification IV. Therefore, we believe that the i.r. spectrum of transpolyalkenamers cannot be completely interpreted on the basis of the single chain vibrations. Strong similarities are also observed between the spectra of modification III of even trans-polyalkenarners and those of trans-polybutadiene (trans-polybutenamer) and of the alternating ethylene-butadiene copolymer (trans-polyhexenamer).