When PAN fibers containing 4% methyl acrylate, 3.2 denier, react with SO 2 at increasing temperature up to 290°C, the reaction is slow below 230°C and rapidly increases thereafter. The effects of up to 20 w/o S have been studied. The incorporation of S leads to an increase in fiber diameter. Reduction of DTA exotherm with increasing S content suggests that the fibers become more oxidation resistant, which is confirmed experimentally. Increase of secondary modulus and decrease of extension to failure values suggest the presence of cross-links; this is also indicated by IR absorption at 1690 cm −1 which may be ascribed to ▪ groups, with S atoms forming intermolecular bridges. Elemental analysis shows that per each incorporated S atom 2.7 to 3.8 H atoms are removed; the O/S ratio approaches 2 or is closer to 1 for the reaction taking place up to 230 or at 290°C, respectively. Above 230°C there is a certain degradation of the fibers, but this is diminished by pre-treatment at 230°C. The IR evidence (behavior of the 2240 and 2190 cm −1 peaks, presence of the 810 and 1590cm −1 absorption, behavior of the 2870 and 2940 cm −1 peaks) indicates that CN groups react slowly at 230°C but rapidly at 290°C, giving rise to a cyclized and aromatized product. Combining low-temperature (230°C) and high-temperature (290°C) SO 2 treatment coupled with higher stretching force, fibers that retain the maximum of their initial carbon content and possess maximum amount of cross-linking, giving rise to highest mechanical properties of carbonized fibers, can be produced.