A study has been made of the relaxation phenomena in three series of poly­urethanes in the neighborhood of their glass transition temperatures and above. The technique of dynamic mechanical relaxation was used in the investigation at various frequencies up to 110 Hz. Differential Scanning Calorimetry (DSC) was also used as a supplement to the mechanical measurements. The series are: (1) H series based on polymerization of hexamethylene diisocyanate and various diols, (2) DP series based on polymerization of 4,4' diphenylmethane diisocyanate and various diols, and (3) 4M series based on polymerization of 4-methyl meta pheny lene diisocyanate and various diols. Four relaxation regions are discernible in the temperature range investigated and these are labelled (1, a, a, and "'• in order of increasing temperature. The following molecular mechanisms are associated with these relaxations: (1) The (1 relaxation (DP and 4M series only) arises from phenyl group motion in the chain backbone, (2) the a relaxation (all series) arises from microbrownian segmental motion associated with the glass transition, (3) the a, relaxation (H series only) is associated with a solid-solid phase transition involving the change from one crystalline modification to an­other, (4) the a, relaxation (H and DP series only) involves rotational or trans­lational motion of the chains in the crystalline phase.