The thermal decomposition of formic acid was studied in dilute aqueous solutions and in the absence of added oxygen at temperatures between 320 and 500 degrees C and pressures between 178 and 303 atm for residence times between 1.4 and 80 s. Under these conditions, the formic acid conversion ranged from 38% to 100%, and the major products were always CO2 and H-2, which indicates that decarboxylation is the preferred reaction path for formic acid decomposition under hydrothermal conditions. CO also appeared as a product, which shows that a dehydration path is available, but the CO yield was always at least an order of magnitude lower than the yields of Co-2 and H-2. The kinetics of formic acid disappearance and product formation at temperatures above 320 degrees C are consistent with a reaction rate law that is first order in formic acid. The implications of the present results to the generally accepted molecular decomposition mechanism are discussed, as are the alternative free-radical, ionic, and surface-catalyzed reaction mechanisms proposed in the literature.