Density functional theory (DFF) and threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) have been used to investigate the dissociation dynamics of the ethylene glycol ion. A total of 13 isomers of the ethylene glycol ion (C2H6O2+.) and the transition states connecting them were obtained at the B3LYP/6-31G(d) level. The TPEPICO experimental results show that the CH3OH2+ ion, produced by a double hydrogen transfer, and the CH2OH+ ion, produced by direct C-C bond cleavage, are the two dominant products. The H2O loss channel, the lowest dissociation energy channel according to the DFF calculations, is suppressed because of a high barrier leading to its formation. The time-of-flight distributions of the CH3OH2+ ion at low energies are asymmetric, which indicates that this ion is produced from a slowly dissociating (metastable) parent ion. A two-well-two-channel model is proposed to describe the isomerization and dissociation process. The simulations combined with RRKM theory suggest that the production of the CH3OH2+ ion involves a hydrogen-bridged reaction intermediate, and its slow production is caused by tunneling through the isomerization barrier, This mechanism is supported by data for deuterated ethylene glycol, The 0 K appearance energy for the CH2OH+ ion is determined to be 11.08 +/- 0.04 eV, from which the 298 K heat of formation of the ethylene glycol molecule is determined to be -383.1 +/- 4.5 kJ/mol, in agreement with other experimental values.