MOLECULES composed of interlocking rings are termed catenanes. Since the first synthesis of a two-ring catenane (a [2]catenane) in 1960(1), these and related interlinked compounds have attracted considerable interest2-10, in part from the perspective of forming molecular devices and nanoscale architectures11,12. Here we report the formation of a [2]catenane from two complete rings, reminiscent of the trick of interlinking ’magic rings’. This supermolecule, 1 in Fig. 1, exists in rapid equilibrium with the monomeric rings 2, as revealed by H-1 NMR and electrospray mass spectrometry. At low concentration of molecule 2, the equilibrium lies towards the monomers, but at higher concentrations the catenane is the overwhelmingly dominant species in solution.