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MD simulations were carried out to examine the partitioning of dissolved gases between clay interlayer water and bulk liquid water. Our results have broader relevance to the adsorption of hydrophobic solutes (including organic molecules) (44) at clay–water interfaces, for example, during sedimentary rock diagenesis, (45) soil formation, (46) and contaminant migration in the subsurface. (42,43) This hydrophobic character is modulated by the presence of exchangeable cations and by the templating of interfacial water by the clay surface. Instead, we find that clay surfaces have a significant hydrophobic character at the atomistic scale, as previously noted in the context of cation-exchange selectivity. (33−41) Our results reveal that dissolved gases do not generally behave as inert tracers in the presence of clay minerals. Our simulation results are consistent with experimental data on the solubilities and diffusion coefficients of gases in bulk liquid water. Here, we present MD simulations and gravimetric adsorption experiments designed to examine the partitioning of gases of interest in groundwater hydrology (CO 2, CH 4, H 2, noble gases) between bulk liquid water and water-saturated clay interlayer nanopores. (15,16) The mechanism of this partitioning remains unknown, as does its relevance to gases other than CO 2. The past few years, however, have yielded increasing evidence that aqueous CO 2 partitions preferentially into the interlayer nanopores of smectite clay minerals, (5,12−14) the main contributor to the specific surface area and nanoporosity of sedimentary rocks. (10,11) Hydrologic studies in these areas invariably assume that dissolved gases behave as inert tracers of fluid migration in water-saturated rocks. In particular, it informs noble-gas geochemistry reconstructions of subsurface fluid migration (1,2) and model predictions of the fate and transport of CO 2, H 2, and CH 4 in carbon capture and storage, (3−5) radioactive waste storage, (6,7) and shale gas extraction, (8,9) three technologies with the potential to contribute roughly half of global CO 2 abatement efforts over the coming decades. The aqueous geochemistry of dissolved gases in sedimentary rocks is a recurrent topic in groundwater hydrology studies. Our results have implications for the fundamental science of hydrophobic adsorption, for the use of dissolved gases as tracers of fluid migration in the subsurface, and for low-carbon energy technologies that rely on fine-grained sedimentary rocks, such as carbon capture and storage, nuclear energy, and the transition from coal to natural gas. Our results indicate that dissolved gases likely do not behave as inert tracers in fine-grained sedimentary rocks such as shale and mudstone, as routinely assumed in groundwater hydrology studies. The affinity of dissolved gases for the clay surface shows significant variations related to the size and shape of the adsorbing molecules and the structuring of interfacial water by clay surfaces. Our results confirm that clay minerals, despite their well-known hygroscopic nature, have a significant hydrophobic character at the atomistic scale. Here, we use molecular dynamics (MD) simulations and gravimetric adsorption experiments to determine the solubilities of CO 2, CH 4, H 2, and noble gases in clay interlayer water. The fundamental basis of this selectivity remains unknown, as does its relevance to other gases. In the past few years, experimental studies have shown that CO 2 is roughly 5 times more soluble in water-saturated clay interlayer water than in bulk liquid water.