Inorganic membranes based on carbon molecular sieve (CMS) films hosting slit-like pores can yield high molecular selectivity with a sub-angstrom resolution in molecular differentiation and therefore are highly attractive for energy-efficient separations. However, the selective layer thickness of the state-of-the-art CMS membranes for gas separation is more than 1 μm, yielding low gas permeance. Also, there is no room-temperature functionalization route for the modification of the pore-size-distribution of CMS to increase the molecular selectivity. In this context, we report two novel fabrication routes, namely, transfer and masking techniques, leading to CMS films with thicknesses as small as 100 nm, yielding attractive gas-sieving performances with H 2 permeance reaching up to 3060 gas permeation unit (GPU). Further, a rapid and highly tunable room-temperature ozone treatment-based postsynthetic modification is reported, shrinking the electron density gap in the nanopores by a fraction of an angstrom and improving gas selectivities by several folds. The optimized membranes yielded H 2 permeance of 507 GPU and H 2 /CH 4 selectivity of 50.7. © 2019 American Chemical Society.