An acid-catalyzed dehydration mechanism was investigated for formic acid decomposition through calculations based on density functional theory. In previous experimental investigations, formic acid dehydration in hot compressed water was reported to proceed faster at pressures >30 MPa. Higher concentration of hydrogen ions because of the large ion product of water at high pressure was believed to contribute to the acceleration of the dehydration reaction. In this study, the structures and energies of the transition states and intermediates were determined through calculations based on density functional theory with the B3LYP/6-311+G(2d,p) level of theory. A comparison of their threshold energies indicated that the dehydration proceeded via the protonation of hydroxyl oxygen, and that the acid-catalyzed dehydration was energetically more favored than the water-catalyzed mechanism. These results suggested that the abundant hydrogen ions in hot compressed water at high pressure accelerated the dehydration occurring via an acid-catalyzed formic acid dehydration mechanism. (C) 2013 Elsevier B.V. All rights reserved.