Porous materials are used extensively in industrial filtration and mass separation processes, but it is often difficult to predict their mass transport behavior because porous materials are an inherently heterogeneous medium and multiple microscopic mechanisms can lead to macroscopic changes in transport. To provide a microscopic view of hindered porous transport, we present the results of single-particle tracking experiments in which We followed the diffusive motion of individual nanoparticles in commercial filtration media. We compared two materials, glass fiber and nitrocellulose, with similar nominal characteristics, but we found that the diffusion behavior of the embedded particles differed significantly. While diffusion in the glass fiber material was nearly unhindered, the dynamics were heterogeneous and significantly slowed in the nitrocellulose. We rationalized the observations based on differences in geometric hindrance, particle binding, and hydrodynamic interactions. Our results highlight the ability of single-particle tracking to differentiate between distinct dynamic mechanisms; and they suggest that nominal material characteristics may be 4 poor predictor of transport properties.