Broadly, we find that the RF model predicts permeabilities in a much narrower space than the DNN ensemble, which explains its lower R 2 values on the test set and further supports the observation that the DNN ensemble is more accurate and generalizable. Predicted permeabilities for each screening dataset lie in their expected region in the permeability-selectivity space, which further supports the accuracy of our ML models. Namely, both models predict permeabilities close to existing Robeson upper bounds for polymers in Dataset D, which consists entirely of ladder polymers (a subclass of PIMs). Similarly, Dataset C consists of polyimides (including many PIMs), and their permeability predictions span a space that includes polymers below and above the Robeson upper bound, reflecting the dataset's diversity. However, Dataset B corresponds to mostly polymers with low permeability and selectivity. We believe that this can be explained by the fact that PoLyInfo is a broad database that contains many polymers that are not suitable for gas separation applications, and Dataset B is populated from existing polymers in PoLyInfo. Most promisingly, the DNN model predicts thousands of polymers from Dataset C to be above the 2008 Robeson upper bound, for O2/N2, CO2/CH4, CO2/N2, and H2/CO2 separations, which is summarized in Table . We further find that the DNN ensemble trained on MFFs not only generalizes but also extrapolates. We discover a class of hypothetical polymers in Dataset C with never-before-seen ultrahigh CO2 permeability (greater than 10 5 Barrer) and a class of polymers with ultrahigh O2 permeability (greater than 10 We select several hypothetical polymers with high predicted permeability and selectivity across all four separations under study, and we validate their performance using MD simulations to calculate permeability. Details of intermediate values calculated during the simulation process can be found in Table of Supporting Information, while SMILES strings for the selected polymers are given in Table . Note that our MD simulation protocol has been benchmarked against experimental and simulation values for the gas permeabilities of PIM-1 , with good agreement with the literature (Table of Supporting Information).