Propylene was polymerized in gas phase and liquid phase by using a novel nonporous Ziegler-Natta-catalyst system. The polymer particles formed at different polymerization times were used for sorption measurements. In both cases it was found that the effective diffusion coefficient is increasing with increasing size of polymer particles and the effective diffusion coefficients of polymer particles formed by liquid-phase polymerization are larger than those of polymer particles produced by gas-phase polymerization. The effective diffusion coefficients of polymer particles are in the range of 2 X 10(-11) to 1.6 X 10(-10) m(2)/s with activation energies from 34 to 22 kJ/mol. The analyzed polymer particles have average diameters between 250 and 875 mu m. The solubility of propylene in polypropylene particles can be described by the law of Henry at conditions studied. (c) 2006 Wiley Periodicals, Inc.