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 angstrom allowing it to readily absorb small molecules such as H(2) and CO(2). ZIF-8/Matrimid (R) 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 (R) MMM show a loss of mechanical strength, leading to a decrease in flexibility. The ZIF-8/Matrimid (R) 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. (C) 2010 Elsevier B.V. All rights reserved.