The structure evolution was studied by mass spectrum (MS), differential scanning calorimetry (DSC) and thermogravimetry (TG), Fourier transform infrared (FTIR) spectroscopy. The results indicated that the C=N and C=C groups appeared gradually with the increase of the temperature in air and nitrogen. The C=O groups appeared because of oxidative reaction in air. The C=N, C=C and C=O groups were all chromophores. The effect of conjugated C=N and C=C on the absorption of the visible light was shifted to longer wavelengths and indicated pi-pi* transition. There was a strong bathochromic effect as the number of C=C bonds were increased. The effect of C=O and -NH2 on the absorption of the visible light was shifted to longer wavelengths and indicated n-pi* transition. Oxygen could facilitate chemical reactions in air. Hence, the color of PAN in air was deeper than in nitrogen at the same temperature. The structural change of PAN in air was faster and more complex than in nitrogen. PAN fibers treated in air turned black after 230 degrees C. However, PAN fibers turned black at 350 degrees C in nitrogen. The MS and FTIR indicated that cyclization occurred before dehydrogenation during stabilization in air and nitrogen. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 462-468, 2010