It has been known that photoionization of ethylene glycol generates protonated methanol when the ionization energy is in the vicinity of the vertical ionization energy. Although two different isomerization paths have been proposed for the protonated methanol production, the isomerization path has not yet been identified. To investigate the isomerization of ionized ethylene glycol, infrared (IR) predissociation spectroscopy based on vacuum ultraviolet photoionization is carried out for neutral and cationic ethylene glycol and partially deuterated isotopomer (HOCD₂CD₂OH). The IR spectroscopic results indicate that ionized ethylene glycol isomerizes to the protonated methanol. HCO complex, and the isomerization path involving the double proton transfer is identified. This isomerization path is also supported by the theoretical isomerization path search, which demonstrates that several reaction pathways are mutually intercommunicated.