Octakis-6-guadinidino-γ-cyclodextrin (gguan) hydrochloride in the presence of phosphates crystallises from aqueous solution in the unprecedented form of a superdimer (dimer-within-a-dimer). The self-assembly exposes four circular octa-guanidinium regions that bind and stabilise discrete H-bonded phosphate anion dimers. The small (∼2 nm) gguan-phosphate assembly is preorganised and stable in aqueous solution, as demonstrated by DLS and NMR experiments.