Self-assembling nanotubes made from H-bonded cyclic D/L peptide (CP) subunits have great potential for the construction of nanomaterials of vast chemical and structural diversity. They have potential applications as drug-delivery vehicles, synthetic membranes and protein/ion-channels but difficulties in structure-characterisation and poor control over nanotube assembly hinder their development into functional nanomaterials. We present the first crystal structures of continuous CP nanotubes with antiparallel and parallel stacking arrangements, assembled separately from two peptides; cyclo[(Asp-D-Leu-Lys-D-Leu)2] and cyclo[(Asp-D-Ala-Lys-D-Ala)2]. Along with X-ray crystallographic studies, the peptide structures were analysed by in-solution and solid-state techniques including DLS, cryo-TEM and FT-IR. Together, these studies provide insight towards the intermolecular interactions between cyclic peptide subunits that will be critical in directing and controlling the self-assembly process to develop complex organic, functionalised materials for application in biomedicine.