Decomposition kinetics of 1.00 m formic acid and sodium formate to form CO2 (and H-2) were probed under hydrothermal conditions of 280-330 degrees C and 275 bar. Flow reactor spectroscopy cells constructed from different metals (316 stainless steel, 90/10 Pt/Ir alloy, and grade 2 Ti) with diamond and sapphire windows were used to determine the rate of formation of CO2 in situ by IR spectroscopy. A single fluid phase was maintained. CO2 was produced at different rates depending on the metal and, in the case of 316 stainless steel, with a lot-dependent, concentration-time profile. The values of E-a = 87-121 kJ/mol resemble those previously reported for surface- and H2O-catalyzed decarboxylation of formic acid. Much higher E-a values are reported for unimolecular decomposition. Sodium formate decarboxylates more slowly than formic acid. In contrast to formic acid, the rate of decarboxylation of malonic acid in all of the cells is the same within error, which is consistent with a homogeneous unimolecular reaction.