We performed configurational-bias Monte Carlo (CBMC) simulations for the adsorption of methane, ethane, propane, butane, and isobutane in MFI-type zeolites with different concentration and nature of nonframework cations. Additional molecular simulations for a mixture of butane/isobutane in these MFI-type zeolites were carried out. The effect of the contentsdensity and naturesof nonframework cations was systematically analyzed, and our results show that even though all of the MFI-type zeolites are structurally similar, differences in the size and concentration of the nonframework cations lead to differences in their adsorptive properties and selectivity performance. For a given cation, the adsorption of alkanes in MFI increases with decreasing the nonframework cation concentration, and for a given Si/Al ratio, the adsorption of alkanes in MFI increases with decreasing atomic weight of the nonframework cation.