Porous additives offer an attractive pathway to enhance the performance of polymeric gas separation membranes. Previously reported porous additives, such as zeolites and metal-organic frameworks, suffer from poor interfacial binding with the polymer matrix, which leads to nonselective gas transport pathways. Porous organic cages (POCs) are an exciting new family of soluble additives that could ameliorate these transport issues by integrating intimately with the polymer matrix. By using Voronoi network analysis, grand canonical Monte Carlo simulations, and molecular dynamics, we provide a theoretical assessment of the benefit of using POCs as additives for mixed matrix membranes (MMMs). We reveal that some MMMs containing POCs exhibit enhanced selectivity and permeability compared to the neat polymer matrix, particularly for H2/CO 2 separations.