Surface sulfonation of solid polypropylene (PP) with hot concentrated H2SO4 has been studied by diffuse reflectance Fourier-transformed infrared (DRIFT) and X-ray photoelectron (XP) spectroscopies. Both measurements indicated that the sulfonation of PP followed by slight oxidation and the formation of C=C bonds is enhanced with both increasing reaction temperature and time. The time dependence of H2O penetration into sulfonated PP mesh sheets was analyzed using a rearranged Washburn’s equation, providing a quantitative indicator of surface hydrophilicity. A positive linear correlation was obtained between the hydrophilicity and the degree of sulfonation. The charge distribution calculated for models of SO3 and PP molecules by the PM3 molecular orbital (MO) method suggested that the sulfonation of Pla proceeds via electrophilic addition of SO3. It was further predicted from the MO calculations for models of sulfonated PP molecules that side CH3 groups are more susceptible to sulfonation than C atoms of the main chain due to a drastic conformational change of the main chain in the latter case. The heat of reaction in each pathway (Delta E) was estimated to be Delta E(side CH3 groups) = -63.5 kJ (unit mol)(-1), Delta E(C(CH3)SO3H) = -56.8 kJ (unit mol)(-1), and Delta E(CH(SO3H)) = -55.8 kJ (unit mol)(-1), respectively. The selectivity in the sulfonation of PP termed "the conformational change restricted selectivity (CCRS)" was shown to increase with a decrease in the reaction temperature by analyzing the DRIFT signals around 600 cm(-1) assigned to the deformation of S-O bonds and the stretching vibration of S-C bonds.