Two series of cyano-substituted polypentenamers and ·their hydrogenated derivatives have been studied by dynamic mechanical and dielectric relaxation in the temperature range from -160 to 110°C and at frequencies from 3.5 Hz to 10 kHz. Series I contains 7.4 mol % cyano groups and series II contains 20 mol % cyano groups. The amorphous cyanopolypentenamers display one major mechanical and dielectric relaxation region asso­ciated with motions accompanying the glass transition and labeled /3. A small secondary relaxation, labeled 'Y, is also discernible in the mechanical E" results. The semicrystalline hydrogenated derivatives exhibit three relaxation regions labeled a, /3, and 'Y in order of decreasing temperature. The a relaxation which occurs in both series I and II mechanically is resolvable by the dielectric technique only in annealed series I samples. The f3 and 'Y relaxations, on the other hand, are present both mechanically and dielectrically in all the derivatives. Enthalpies of fusion for the semicrystalline derivatives were determined using the diluent technique. It was found that the enthalpy of fusion depends on the concentration of the cyano substituents. Available evidence indicates some degree of incorporation of the cyano groups into the crystal lattice but this incorporation is considerably less than that occurring in chlori­nated polyethylenes. The results indicate that the a relaxation arises from motions within the crystal phase, the f3 relaxation from motions accompanying the glass transition, and the 'Y relaxation from local motions in both the crystal and amorphous phases.