The effect of sulfonation treatment was investigated on the molecular structure and mechanical properties of isotactic polypropylene fibers extruded at a take up speed of 2500 m/min. It was found that at extensive sulfonation times, the sulfonated structure showed the characteristic features of carbonized structure as indicated by the results of the density and the X-ray diffraction measurements. Mechanical properties of the sulfonated samples were found to be adversely affected by the sulfonation conditions. Scanning electron microscopy observations showed surface irregularities at low sulfonation times and fiber fractures at high sulfonation times. Polarized infrared spectroscopy measurements analysed by curve fitting procedure showed increasing molecular orientation of long helical chain segments represented by the IR band at 841 cm(-1) whereas amorphous structure represented by the IR band at 2723 cm(-1) showed gradual loss of orientation with the progress of sulfonation. IR bands assigned to the sulfonic acid groups formed during sulfonation treatment showed perpendicular polarization and low molecular orientation characteristics indicating the initiation and the development of crosslinking process being perpendicular to the fiber axis direction. Analysis of the equatorial X-ray diffraction traces showed the loss of crystallinity where the paracrystalline phase disappeared faster than the crystalline alpha-monoclinic phase. During the sulfonation treatment, content of amorphous phase showed gradual increase in line with decreasing crystallinity. In accordance with the loss of crystallinity, apparent crystallite sizes corresponding to the 110, 040 and 130 planes of the alpha-monoclinic phase also decreased gradually with increasing sulfonation time. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 3375-3389, 2012