The central OCCO backbone of the 1,2-ethanediol molecule adopts the gauche conformer in the gaseous and crystalline states but exists in conformational equilibrium between gauche and trans in the liquid; an observation that has been attributed to the competition between intra- and intermolecular interactions. Here, we show that the nuclear Overhauser effect (NOE) has the ability to distinguish inter- from intramolecular interactions in liquid 1,2-ethanediol. We do so by exploiting the secondary isotope effect to distinguish the hydroxyl protons of HOCH2CH2OH and the deuterated HOCD2CD2OH in the H-1 NMR spectra of mixtures of the two and, in conjunction with ab initio MD simulations, show how the interplay between inter- and intramolecular interactions gives rise to the conformational isomers in the liquid state of 1,2-ethanediol.