In dilute aqueous solutions, D-glyceraldehyde exists both in the free and hydrated monomeric forms; the equilibrium between them depends on the temperature. The specific rotation of the equilibrium mixture was measured polarimetrically in the temperature range from 20 to 80 °C, the concentrations of both forms being determined by 1H NMR spectroscopy. From these data, the specific rotations of both forms of D-glyceraldehyde in the indicated temperature range were calculated.