AbstractExisting dynamic mechanical relaxation data on two polymers containing aromatic groups in the backbone chain,viz., polyimide PI, and poly(2,6‐dimethyl‐p‐phenylene oxide), PDMPO, are reviewed and new data bearing on specimen purity and water content are presented. It is demonstrated that three separate relaxation processes are normally present in both polymers in the temperature range from about 100–400°K. The lowest temperature relaxation, designated γ, occurs in the 100–180°K range and is considered to arise from limited rotational oscillation of some phenylene rings about the 1,4 axis. Another relaxation process, designated β arises from the presence of water in the polymer. Its strength increases with increase of water content, its activation energy is about 10–12 kcal/mole, and it occurs in the 180–190°K range at 1 Hz. A higher strength secondary glass relaxation process, designated β*, occurs in the 280–400°K range at usual measuring frequencies (1 Hz to 104Hz). This relaxation is thought to be associated with the combined reorientational motion, of rings and connecting atoms of the monomer unit in “defect” regions, or regions of poor chain packing. As such, its position is affected by sample history. Both polymers also show a rise in damping in the high temperature region indicative of additional main chain motions. In addition to the above relaxations, PDMPO shows a δ‐relaxation below liquid nitrogen temperatures, associated with onset of Me rotation and PI shows an additiona