A helical aromatic oligoamide foldamer encapsulates tartaric acid with exceptional affinity, selectivity, and diastereoselectivity. The structure of the complex has been elucidated both in solution by NMR spectroscopy and in the solid state by X-ray crystallography, making it possible to rationalize the strong effects observed, particularly the role of hydrogen bonds between the hydroxyl and carboxylic acid groups of tartaric acid and the inner wall of the helically folded capsule, which completely surrounds the guest and insulates it from the solvent.