Molecular recognition of carbohydrates plays an important role in nature.The aggregation of the smallest sugar, glycolaldehyde,w as studied in ac onformer-selective manner using high-resolution rotational spectroscopy. Tw od ifferent dimer structures were observed. The most stable conformer reveals C 2 -symmetry by forming two intermolecular hydrogen bonds,giving up the strong intramolecular hydrogen bonds of the monomers and thus showing high hydrogen bond selectivity.Byanalyzing the spectra of the 13 Cand 18 Oisotopologues of the dimer in natural abundance,w ec ould precisely determine the heavy backbone structure of the dimer.Comparison to the monomer structure and the complex with water provides insight into intermolecular interactions.D espite hydrogen bonding being the dominant interaction, precise predictions from quantum-chemical calculations highly rely on the consideration of dispersion.