Diffusion coefficients of propylene and ethylene through particles and films of isotactic polypropylene (iPP) as well as through films of atactic, completely amorphous, polypropylene have been measured by gravimetric experiments. iPP particles with relatively similar crystallinities but with different particle size distributions (PSD), and different origins have been used. They exhibited very different diffusion coefficients when the average radius of the PSD was used as the dimensional parameter of the diffusion process. However, films prepared with these particles and having similar crystallinities provided similar diffusion coefficients. The results are consistent with the idea of a multigrain structure in the particles, where the apparent (or experimental) diffusion coefficient is a consequence of the complex system formed by internal and external areas. The diffusion coefficient of the films, taken as the real diffusion magnitude, can be used in the calculation of the true diffusional radius of these multigrain particles. Experimental results of the diffusion coefficients have been reasonably well reproduced with the aid of the Vrentas-Duda free volume model. (c) 2007 Wiley Periodicals, Inc.