Multinuclear pulsed field gradient (PFG) NMR was used to study the self-diffusion of carbon dioxide and methane as equimolar mixtures and pure gases inside ZIF-11 crystals. Diffusion measurements were performed under conditions where the length scales of displacements were comparable with or smaller than the mean size of ZIF-11 crystals. It was found for both carbon dioxide and methane that the intracrystalline diffusivity decreases with an increasing diffusion time. Quantitative analysis of these time dependencies using a previously reported model explained the dependencies by the reflections of gas molecules from the external crystal surface. This analysis also allowed the intracrystalline gas diffusivities that are not perturbed by the influence of the external crystal surface to be determined. The ratio of such true intracrystalline diffusivities for CO2 and CH4 is found to be greater in the mixed sorbate sample than in the single-sorbate samples when the total gas concentration in each sample was the same within experimental error.