Investigations of the 39-cm-1 band observed at low temperatures in the far-infrared region suggest that this band is characteristic of chain-extended polyethylene (CEPE). In the process of chain extension via the hexagonal phase, partial disclinations (long chain twists) and gauche bond defects are most certainly introduced. During the transition from the hexagonal to the orthorhombic phase upon cooling, these defects can be “frozen in” and activate the spectroscopically inactive modes of the PE chain and lattice by a lowering of the molecular symmetry in the vicinity of the defect. This mechanism is believed to be the origin of the IR band at 39 cm-1. Spectroscopic studies do not support the assignment of this band to the monoclinic phase known to be present in mechanically processed PE. © 1984, American Chemical Society. All rights reserved.