We have carried out a neutron powder diffraction study of deuterated ethylene glycol (1,2-ethanediol), and deuterated ethylene glycol monohydrate with the D2B high-resolution diffractometer at the Institut Laue-Langevin. Using these data, we have refined the complete structure, including all hydrogen atoms, of the anhydrous phase at 220 K. In addition, we have determined the structure of ethylene glycol monohydrate at 210 K using direct space methods. Anhydrous ethylene glycol crystallizes in space-group P212121 with four formula units in a unit-cell of dimensions a = 5.0553(1) Å, b = 6.9627(1) Å, c = 9.2709(2) Å, and V = 326.319(8) Å3 [$\rho _{{\rm calc}}^{{\rm deuterated}}$ρ calc deuterated = 1386.26(3) kg m−3] at 220 K. Ethylene glycol monohydrate crystallizes in space-group P21/c with four formula units in a unit-cell of dimensions a = 7.6858(3) Å, b = 7.2201(3) Å, c = 7.7356(4) Å, β = 92.868(3)°, and V = 428.73(2) Å3 [$\rho _{{\rm calc}}^{{\rm deuterated}}$ρ calc deuterated = 1365.40(7) kg m−3] at 210 K. Both the structures are characterized by the gauche conformation of the ethylene glycol molecule; however, the anhydrous phase contains the tGg′ rotamer (or its mirror, g′Gt), whereas the monohydrate contains the gGg′ rotamer. In the monohydrate, each water molecule is tetrahedrally coordinated, donating two hydrogen bonds to, and accepting two hydrogen bonds from the hydroxyl groups of neighboring ethylene glycol molecules. There are substantial differences in the degree of weak C−D···O hydrogen bonding between the two crystals, which calls into question the role of these interactions in determining the conformation of the ethylene glycol molecule.