The unimolecular reactions of ionized ethyl glycolate, HOCH2CO2C2H5•+, and its enol isomer, HOCH=C(OH)OC2H5•+, have been studied by a variety of techniques including 2H-, 13C- and 18O-labelling experiments, kinetic energy release information, analysis of collision-induced dissociation and neutralization–reionization mass spectra and thermochemical measurements. The metastable ionized enol eliminates C2H4 essentially exclusively by β-hydrogen transfer to give (HO)2C=CHOH•+, which itself expels H2O with very high selectivity (∼99%). The metastable ionized keto isomer also eliminates C2H4, but minor amounts (∼5%) of competing fragmentations resulting in expulsion of H2O, HOCO• and HCO• are also observed. Moreover, in this case, loss of C2H4 no longer involves specific β-hydrogen transfer. This behavior is interpreted in terms of rearrangement of the ionized keto form via a 1,5-H shift to the distonic ion HOCH2C+(OH)OCH2CH2•, which undergoes a further 1,5-hydrogen shift to form HOCH=C(OH)OC2H5•+, from which C2H4 is lost to give (HO)2C=CHOH•+. The chemistry of these C4H8O3•+ species, in which unidirectional tautomerism of HOCH2CO2C2H5•+ to HOCH=C(OH)OC2H5•+ via two 1,5-H shifts is important, contrasts sharply with the behavior of the lower homologues, HOCH2CO2CH3•+ and HOCH=C(OH)OCH3•+, for which the analogous tautomerism via 1,4-H shifts does not occur. The mechanism for loss of C2H4 from HOCH=C(OH)OC2H5•+ is essentially the same as that observed for ionized phenyl ethyl ether in that the reaction proceeds via an exothermic proton transfer in an ethyl cation–radical complex. Neutralization–reionization experiments show that neutral trihydroxyethylene in the gas phase is a remarkably stable species which does not tautomerize to glycolic acid, HO–CH2–COOH. Using G2(MP2) theory, the enthalpy of formation, ΔHf298, of the neutral molecule (most stable conformer) was found to be −449.4 kJ mol−1 while that of the corresponding ion was calculated to be 299.6 kJ mol−1.