An unsteady-state diffusion model is developed to calculate the transport of penetrant molecules in semicrystalline nonporous polyolefin films and porous powders in terms of the internal particle morphology (i.e., pore size distribution and crystallinity) of the polymer. To calculate the diffusion coefficient of penetrant molecules in a semicrystalline nonporous polymer matrix, the free volume theory is employed. Moreover, to calculate the diffusion coefficient of penetrant species in a porous polymer matrix, a dual diffusion model, accounting for the molecular diffusion of penetrant in the pores and the amorphous polymer phase, is developed. A novel experimental setup, comprising a gravimetric magnetic suspension microbalance connected in series with an optical view cell, is employed to carry out dynamic sorption/swelling experiments of ethylene and propylene in both nonporous films and porous high-density polyethylene (HDPE) powders. Experimental sorption measurements, along with the developed unsteady-state diffusion model, are then employed to calculate the diffusion coefficient of alpha-olefins in porous and nonporous semicrystalline HDPE as well as the overall porosity of HDPE powders. Finally, to reconstruct the pore size distribution in a porous polyolefin particle in terms of the estimated overall particle porosity and the size distribution of the microparticles, a diffusion-limited aggregation (DLA) model is developed. It is shown that the morphological characteristics of porous polyolefin particles (i.e., porosity and pore size distribution) can be described using the proposed DLA model in terms of the size distribution of the microparticles and the extent of microparticles fusion.