The isothermal oxidative stabilization of polyacrylonitrile fibers has been carried out at 210, 230, and 250 degrees C. The stabilized fibers, treated for different times, have been characterized with elemental analysis, wide-angle X-ray diffraction, optical microscopy, and field emission scanning electron microscopy. A parabola relationship has been established between the oxygen uptake and stabilization time, whereas the aromatization index shows a trend of moderate ascension, retention, and acceleration. With increasing temperature and time, the skin-core morphology of the stabilized fibers becomes more and more distinct, but the skin thickness is almost unchanged for 60 and 120 min at 250 degrees C. The fracture mechanism is ductile fracture in the core but is brittle fracture in the skin. The results indicate that the initial rapid oxygen uptake at a high temperature and the subsequent intense aromatization are responsible for the formation of the skin-core morphology. On the basis of the isothermal stabilization, an onion-like model is proposed for the structure of stabilized fibers that are treated by stepwise increasing temperatures in industrial production. (c) 2007 Wiley Periodicals, Inc.