Ammonium-modified polyacrylonitrile (PAN) dope was prepared by free-radical solution copolymerization of acrylonitrile (AN) and ammonium itaconate (AIA), adding different weight percent amount of sulphur into quantitative dope and then the dope was spun into precursor fibers. Effects of sulphur on the characteristics of the copolymers and precursor fibers even the oxidated fibers or the resultant carbon fibers were studied. It has been found that the adding sulphur could obviously increase the hydrophilicity of the copolymer. But in the spinning processes, with the concentration of the adding sulphur increasing, the apparent minus jet stretch of as-spun fibers varied from -21% to -66%, the crystallinty increased from 21.2% to 40.5%, the void volume, fineness, tenacity and elongation at-break of the ammonium-modified precursors all decreased, which could result in the increase of the comprehensive properties of the resultant precursors. It was testified that the precursors developed from the copolymer with the higher concentration of adding sulphur have higher degree of orientation. The resultant carbon fibers developed from precursors of the adding sulphur show an improvement in tensile strength of about 30%. The activation energy of cyclization can be reduced from the original 150 kJ mol(-1) to about 110 kJ mol(-1). (c) 2007 Wiley Periodicals, Inc.