In molecular separation processes based on nanoporous materials, microscopic transport properties are key to the kinetics of the adsorption process at the macroscopic scale. Two distinct use cases for nanoporous materials in separation processes exist: adsorption-based separation, which is primarily a thermodynamic process, and nanoporous separation membranes, which rely on both kinetic and thermodynamic properties. Depending on the targeted application, diffusion is either the main performance metric (for membranes) or a secondary parameter (for adsorption) that is often overlooked. In processes based on molecular sieving, for instance, gases are passed through a membrane material that selectively blocks certain atoms or molecules on the basis of their size, while allowing other particles to diffuse freely. In the steady state, the performance of the separation is, in part, related to the ratio of diffusion coefficients for the species involved. On the other hand, the thermodynamic selectivity is the primary performance metric in adsorption-based separation processes commonly performed at the industrial scale, such as pressure and/or temperature swing adsorption (PSA, TSA or PTSA). However, even in those cases, it is worth considering that the kinetic performance can enhance the overall industrial process. For instance, in breakthrough experiments used to characterize the comparative adsorption performances of a gas mixture -and somewhat akin, at the lab scale, of a pressure swing adsorption -the shape of the breakthrough curve can be explained by diffusion processes.