The thermal behavior and structural evolution during the thermal stabilization of polyacrylonitrile (PAN) fibers in N-2 and air were investigated using differential scanning calorimetry and solid-state C-13 nuclear magnetic resonance. It was found that an oxidation reaction, that generated carbonyl (C=O) groups could occur at 160 degrees C which has not been reported in the literature. It is proposed that the cyclized structures in the PAN macromolecule chains are a prerequisite for the oxidation. Further investigations indicate that with more cyclized structures in the PAN macromolecule chains, the oxidation proceeds more readily, which is consistent with the proposed mechanism. The kinetic parameters for the oxidation and cyclization reactions were estimated using the Kissinger method. The activation energies for the reactions of oxidation and cyclization for PAN fibers are about 96.4 kJ/mol and 190.0 kJ/mol, respectively, which implies that the cyclization is the rate determining step during the thermal stabilization of PAN fibers. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 3198-3203, 2013