Density functional theory calculations of glyceraldehyde conversion were performed to elucidate the relative reactivity between dehydration and retro-aldol reaction. In this study, structures and energies of the reaction system were determined by B3LYP/AUG-cc-pVDZ level of theory to explore the catalytic role of water molecule. The water-assisted retro-aldol reaction is the most favorable pathway among glyceraldehyde conversion. Water molecule participation in the reaction system lowered the threshold energies for both reaction pathways, suggesting that a water molecule acts as homogeneous catalyst. Comparing the threshold energy values between non-water and water-assisted mechanisms, the threshold energy difference in dehydration was found to be greater than that in the retro-aldol reaction. This large catalytic water influence on the dehydration means that the glyceraldehyde dehydration is more enhanced at fairly higher density conditions. The reaction analysis in terms of water molecule participation can explain the influence of supercritical water density upon the glucose conversion and the catalytic role of water molecule in molecular level. (C) 2014 Elsevier B.V. All rights reserved.