The mono deuterium equilibrium isotope effect (EIE) for the exchange of hydroxyl deuterons and protons on 2-hydroxyl and 4-hydroxyl of 2, 4-dihydroxybenzaldehyade were calculated theoretically and also measured with NMR. The equilibrium constant (K-eq = 1.108) was predicted using the geometry and vibrational force constants derived from electronic structure theory followed by the program THERMISTP, which is highly consistent with the NMR results (experimental K-eq = 1.081 and K-eq = 1.066), indicating the formation of the strong hydrogen bond between 2-hydroxyl and the aldehyde group.