Polyolefins are usually made in the form of particles, with the catalyst distributed inside these particles and monomers in gas or liquid form on the outside. Mass transfer restriction of monomers and other molecules into the polymer particles could be of significance. A method of studying this restriction is shown in this article. This method is based on a novel two-phase (pore phase and polymer phase) transport theory developed here, with a dimensionless number for the competitiveness of pore vs. polymer phase transport as the key parameter. Data of mass transfer from polymer particle drying experiments are, by means of this dimensionless transport parameter, generalized so that diffusion resistance during the preceding polymerization of these particles can be calculated. It is believed that this has not been attempted before. This method is used on an example of ethylene diffusion resistance when making polyethylene (PE). The calculated result is that diffusion resistance is significant for slurry polymerization in this case.