Silicalite membranes exhibit an interesting selectivity for pure CO2, which flows through the material faster than He or H2, notwithstanding the larger kinetic diameter of CO2 [58]. The diffusion of CO2 and N2 in silicalite was studied using QENS, with combined coherent QENS-MD used to determine diffusivities for CO2 and more traditional QENS used to determine the N2 diffusivity [59, 60]. The work obtained the diffusivity as a function of intra-crystalline occupancy. A direct comparison between computed and measured transport-diffusion allowed a better understanding of the molecular factors governing the occupancy dependence of the corrected diffusivity, a “pure” kinetic parameter that is largely free of the influence of the isotherm. The corrected diffusivity has often been assumed to be independent of sorbate concentration and approximately equal to the self-diffusivity in the limit of zero occupancy, an assumption that was reassessed in light of these results. These measurements pointed to a significant difference in the occupancy depen­dence of the corrected diffusivity for N2 and CO2, where at 300 K the corrected diffusivity for CO2 was found to decrease with guest loading of the host and for N2 at 200 K it was fairly constant, exhibiting a weak maximum. At the practical level this work found that interactions are considerably stronger (more attractive) for CO2 than for N2 in silicalite, explaining the strong preference of the material for CO2.