The linear low-density polyethylene (LLDPE) fibers were sulfonated in 95%concentrated sulfuric acid.The structure composition and thermal stability of the polyethylene (PE) fibers sulfonated for various times were determined by Fourier transform infrared spectroscopy (FTIR), resonance Raman spectra, elemental analysis,X-ray Photoelectron Spectroscopy (XPS) and thermogravimetric analysis (TGA).It was confirmed that three stages of reaction including substitution, desulfonation and oxidation took place in succession during the sulfonation.The initial increase of sulfonic acid and hydroxyl groups, as an indication of the substitution reaction, led to a proposal that concentrated sulfuric acid at elevated temperatures could give HSO3· and HO·radical which accounted for the sulfonating and alcoholizing of LLDPE.The subsequent decrease of hydroxyl groups and generation of carbonyl groups were attributed to the oxidation reaction.The formation of C=C double bonds and the decrease of sulfur and sulfonic acid groups later on implied the desulfonation reaction.The cross-linked conjugated polyenes structure derived from the desulfonation probably made the sulfonated LLDPE fibers carbonizable.