Oversolubility, which corresponds to large apparent gas solubilities in liquids confined in nanoporous solids, has been proposed as a means to develop novel adsorption, phase separation, or catalytic processes. We report a molecular simulation study to help design such hybrid adsorbents consisting of a solvent confined in host nanoporous materials. Water is selected as the confined solvent because of its ubiquity in industrial applications, while N₂, CH₄, and CO₂ are selected as they allow probing the effect of specific molecular interaction and polarity. For each system, we consider a zeolite, an ordered mesoporous silica, and a metal–organic framework as the confining host as they present different morphologies, porosities, and surface chemistries. We show that oversolubility is always observed because the apparent solubility in these materials surpasses the bulk solubility. In all cases, oversolubility is an enhanced bulklike solubility in which solubility is favored in the regions of low water density formed by the layering of the solvent. Such an oversolubility mechanism, which arises from the fact that the gas–solid interactions are weaker than the solvent–solid interactions, leads to large uptakes as high as a few hundred times that expected from bulk solubility.