<p>Directional mass transport in confined space is crucial to life and the water–energy–environment nexus. Despite progress in understanding biological and designing artificial ionic diodes at the atomic scale, rectifying charge-neutral molecular flow remains a challenge. Here we explore gas transport through an ångström-sized Janus aperture in graphene, which is created by feedback-controlled ozone etching and features oxygen-containing functional groups asymmetrically distributed around the edge. Ten representative gases with molecules of varying compositions, shapes and sizes were measured. The permeation coefficients indicate energy barrier-controlled transport. Rectified flow was consistently observed for seven different species including krypton, xenon, hydrogen, oxygen, nitrogen, carbon dioxide and nitrous oxide, with rectification ratios of up to two orders of magnitude for oxygen. We also performed high-throughput density functional theory calculations, obtaining energy barriers that vary distinctly as the flow direction is flipped, in agreement with experimental measurements and ab initio molecular dynamics simulations. We reveal the impact of the molecular polarizability on the rectified gas flow, while the important role of dipole and higher-order moments remains to be elucidated.</p>