A two-step process has been invented to prepare sulfonated polypropylene from chlorinated polypropylene via thiolation and successive oxidation to enhance the dyeability of polypropylene. With a short thiolation reaction time of 3 h in an N-methyl-2-pyrrolidone solution, 1.7-20.5% sulfur can be incorporated into a polypropylene bulk effectively. Chlorine-thiol substitution and hydrosulfide conversion have been examined with elemental analysis, and their behaviors as a function of the SH/Cl ratio can be explained with an equilibrium model of hydrosulfide and accessible chlorine in a given timescale. Oxidation of thiol has been performed successfully with hydrogen peroxide. The evolution of oxidation intermediates such as sulfoxide, sulfone, sulfinic acid, and sulfonic acid can be identified by Raman and Fourier transform infrared analyses. Sulfonated polypropylene can be stained by a basic dye very effectively, and its dye uptake reaches 190 mmol of dye/kg of polymer for 3.6 mmol of sulfur/g of polymer. This dye uptake is 20 times more effective than that of chlorinated polypropylene on a molar basis. Thus, it is clear that a modification can be performed effectively to enhance the dyeability of polypropylene. (C) 2008 Wiley Periodicals, Inc.