A novel supramolecular nanostructure formed by the coadsorption of the complementarynucleobases guanine (G) and uracil (U) at the liquid (1-octanol solvent)/solid (graphite) interface is revealedby scanning tunneling microscopy (STM). The GU supramolecular structure is distinctly different from thestructures observed by STM when the individual nucleobases (NB) are adsorbed on graphite in the controlexperiments. Using a systematic methodology and ab initio density functional theory (DFT), an atomisticstructural model is proposed for the supramolecular coadsorbed GU structure, which consists of a periodicrepetition of cyclic units based on the strongest GU base pairing.