AbstractNitric acid etching studies have been conducted on samples of commercially available highly oriented gel‐spun polyethylene fibers: SpectraTM 900 and 1000 from AlliedSignal Inc. The results show that the acid attacks the fiber, increases crystallinity as observed by X‐ray diffraction, increases the enthalpy of melting of unconstrained fibers by 13–25%, and removes topological constrains thus facilitating the crystallization of chain segments in the noncrystalline regions. The acid functionalizes the fiber, creating various oxygen and nitrogen‐containing moieties, specifically (CO),CC, and NO2 groups. The small weight loss upon etching, less than 2%, and the fact that fibers weaken but do not fragment suggests that the gel‐spun and ‐drawn morphology is more resistant to acid attack than are other morphologies of polyethylene. Photomicroscopy shows that acid etching opens cracks normal to the fiber axis. That the acid can attack the gel‐spun fibers indicates the presence of structural imperfections such as folds, molecular kinks, or uncrystallized regions within the fibers. Infrared analysis on virgin fibers shows the absence of absorption bands normally associated with the presence of chains lying within amorphous regions in polyethylene. These and other morphological features are integrated into a description of the structure of commercial gel‐spun polyethylene fibers. © 1995 John Wiley & Sons, Inc.