Kinetics of the heterogeneous sulfonation of polystyrene (PS) beads using gaseous SO3 Was investigated. Scanning electron microscopy (SEM) and electron diffraction scattering spectroscopy (EDS) was employed to study the kinetics of diffusion of SO3 into the PS particles. The diffusion of SO3 through the barrier of sulfonated polystyrene (SPS) on the beads surface was the primary parameter determining the rate and the yield of the sulfonation reaction. The measurement of the time dependence of the thickness of sulfonated layer formed on the solid PS surface provided for the hypothesis that the sulfonation in heterogeneous phase was diffusion controlled. Diffusion coefficients of SO3 in PS at -5 degrees C, 22 degrees C, and at 50 degrees C and activation energy of SO3 diffusion to the solid PS were determined from these experimental data assuming in the first approximation a simple diffusion unaffected by the ongoing sulfonation reaction. The experimental data were fitted using Johanson-Mehl-Avrami-Jerofyeev-Kolgomorov's equation to obtain an overall rate constant of heterogeneous sulfonation on solid PS surface. POLYM. ENG. SCI., 49:1839-1845, 2009. (C) 2009 Society of Plastics Engineers