Zeolitic imidazolate frameworks (ZIFs), that have the potential for gas separation, were used as additives in mixed-matrix membranes (MMMs). ZIF-8, which exhibits the sodalite topology, is composed of zinc (II) ion clusters linked by imidazolate ligands. The ZIF-8 pore aperture is 3.4 Å allowing it to readily absorb small molecules such as H 2 and CO 2 . ZIF-8/Matrimid ® MMMs were fabricated with loadings up to 80% (w/w), which are much higher than the typical loadings achieved with select zeolite materials. Only at the highest loading did the ZIF-8/Matrimid ® MMM show a loss of mechanical strength, leading to a decrease in flexibility. The ZIF-8/Matrimid ® MMMs permeability properties were tested for H 2 , CO 2 , O 2 , N 2 , CH 4 , C 3 H 8 , and gas mixtures of H 2 /CO 2 and CO 2 /CH 4 . The permeability values increased as the ZIF-8 loading increased to 40% (w/w). However, at higher loadings of 50% and 60% (w/w), the permeability decreased for all gases, and the selectivities increased consistent with the influence of the ZIF-8 additive. The ideal selectivities of gas pairs containing small gases, such as H 2 /O 2 , H 2 /CO 2 , H 2 /CH 4 , CO 2 /CH 4 , CO 2 /C 3 H 8 , and H 2 /C 3 H 8 , showed improvement with the 50% (w/w) ZIF-8 loading, demonstrating a transition from a polymer-driven to a ZIF-8-controlled gas transport process. In control experiments using as-synthesized ZIF-8 with filled pores, there was no transition at 50% (w/w) loading. This may be the first example of an MMM wherein molecular sieving is evident and suggests that additive loadings >50% (w/w) may be required to observe this effect in MMMs.