Dihydroxyfumaric acid (DHF) is an important organic acid found in vegetal and living cells, a proven intermediate in the cycles of di– and tricarboxylic acids, and in the glyoxalic acid cycle via the tartaric acid transformation cycle. Eschenmoser suggested [1] that glyoxylate and its dimer, DHF, could have served as the basic raw material for the synthesis of organic macromolecules in the constraints of prebiotic chemistry, thus leading to the formation of life on Earth. This, in correlation with earlier results that showed that DHF and some of its derivatives may be successfully used for the enhancement and preservation of wines [2], as inhibitors of nitrosoamines formation in vitro [3,4] and in vivo [5], as well as efficient scavengers of DPPH and ABTS free–radicals [6], prompted us to carry on a series of computational studies, using the ORCA software [7]. A total of 45 isomers of DHF, including 23 keto and 22 enediol forms, were identified and their geometrical isomerization was studied at the B3LYP level of theory using the 6–311++G(2df,2p) basis set in the gas phase, and aqueous solution (SMD model). Three most stable enediol structures were identified. Interconversions between the enediol forms and the keto forms proceed through two paths: (1) proton transfer (≈ 135–160 kJ mol-1) and (2) internal rotation (≈ 0.15–75 kJ mol-1). Furthermore, equilibrium constants have been calculated, along with the forward and reverse reaction rates for the isomerization reactions of the three most stable enediol structures, in gas and water. The DFT method was further used to investigate the keto-enol tautomerism of the most stable three enediol structures of DHF, in the gas phase, and aqueous solution (SMD model). It was found that the activation energy and the free activation energy are in the range of 230–310 kJ mol-1 for the gas phase and by 50-80 kJ mol-1 lower in water, and transition states structures reveal that the carboxylic oxygen that forms the hydrogen bond in the enediol structure is involved in the mechanism of proton transfer. Furthermore, equilibrium constants have been calculated, along with the forward and reverse reaction rates. This research is important mainly because enols are essential intermediates in many important reactions of carbonyl compounds, as well as in several biological reactions.