Artificial synthetic molecules able to adopt well-defined stable secondary structures comparable to those found in nature ("foldamers") have considerable potential for use in a range of applications such as biomaterials, biorecognition, nanomachines and as therapeutic agents. The development of foldamers with the ability to bind and encapsulate "guest" molecules is of particular interest; as such an ability is a key step toward the development of artificial sensors, receptors and drug-delivery vectors. Although significant progress has been reported within this context, foldamer capsules reported thus far are largely restricted to organic solvent systems, and it is likely that the move to aqueous conditions will prove challenging. Toward this end, we report here structural studies into the ability of a recently reported water-soluble self-assembled foldamer helix bundle to encapsulate simple guest molecules within an internal cavity. Seven high-resolution aqueous crystal structures are reported, accompanied by molecular dynamics and high-field NMR solution data, showing for the first time that encapsulation of guests by a complex self-assembled foldamer in aqueous conditions is possible. The findings also provide ample insight for the future functional development of this system.