AbstractA reaction route accounting for the formation of levulinic acid from 5-hydroxymethylfuran-2-carbaldehyde was deduced on the basis of the mechanism previously offered by Horvat, to match the steps with more details. A newly deduced reaction route was proposed between two intermediate products within this mechanism, and the probabilities of the two mechanisms were compared by Gaussian 03 software. It was found that the conversion from the intermediate 2,3-dihydroxy-5-methyl-2,3-dihydro-furan-2-carbaldehyde (DMDFC) to 2,5-dioxo-hex-3-enal (DOHE) in the original mechanism has a lower net energy barrier than that in the newly deduced mechanism, and thus should be more preferred. The mechanism indicates that DMDFC is first protonized, followed by a proton shift process, and thereafter an OH- ion is added, completing the hydration process. Thereafter, an intramolecular H-shift reaction proceeds, leading to conversion to the intended intermediate product DOHE by the consecutive processes of isomerization and dehydration.