Inverse gas chromatography has been used to estimate the characteristic mass transfer time for diffusion of ethylene and propylene in polypropylene particles. It was found that the mass transfer time corresponded to a length scale that is the same order of magnitude as the particle diameter, d(p) (between 0.3 and 1 mum). Furthermore, when interpreted in terms of diffusion coefficients for spherical particles, these coefficients were found to be of the same order of magnitude and those measured elsewhere for diffusion in pure polymer. This implies that diffusion in the particles is controlled by diffusion in the polymer, and that previous descriptions of particle morphology used in modelling studies need to be revised. In addition, experimental evaluation of reaction rates has demonstrated that changing length scales due to particle agglomeration can also significantly reduce the observed activity in gas phase olefin polymerisation, and that the severity of the reduction in rate correlates with the severity of the agglomeration process.