A study has been made of the dynamic mechanical relaxation behavior of a 97% cis polypentenamer and its hydrogenated derivatives. The starting polypentenamer is wholly amorphous and derivatives were synthe­sized having melting points from -11 to 131 °C and degrees of crystallinity from 2 to 80%. Three relaxations are dis­cernible in the temperature range from -160 °C to the melting point and these are labeled a, {3, and 1' in order of de­creasing temperature. The a relaxation occurs only in the crystalline derivatives. In the derivatives of low crystallini­ty, it originates from motions accompanying melting; in derivatives of high crystallinity, it originates from a combina­tion of intracrystalline motions and interlamellar slip. The f3 relaxation arises from microbrownian motion accompa­nying the glass transition. It decreases in temperature initially as double bonds are converted to single bonds but shows an abrupt rise in temperature at the first trace of crystallinity and continues to rise thereafter. The 1' relaxa­tion is absent in the starting polypentenamer but is present in all the derivatives. It increases in magnitude with in­creasing CH2 sequence length and with crystallinity. It arises from a crankshaft motion in the amorphous phase in­volving a few CH2 units and motion in the crystal phase probably at chain end defects. It is suggested that the two­phase model is only applicable to polymers with amorphous phases of sufficiently different structure from the crys­talline phases as to preclude the crystallization of the amorphous phase under any circumstances.