The decomposition of formic acid is studied in a continuous sub- or supercritical water reactor at temperatures between 300 and 430 degrees C, a pressure of 25 MPa, residence times between 4 and 65 s, and a feedstock concentration of 3.6 wt%. In situ Raman spectroscopy is used to produce real-time data and accurately quantify decomposition product yields of H-2, CO2, and CO. Collected spectra are used to determine global decomposition rates and kinetic rates for individual reaction pathways. First-order global Arrhenius parameters are determined as log A (s(-1)) = 1.6 +/- 0.20 and E-A = 9.5 +/- 0.55 kcal/mol for subcritical decomposition, and log A (s(-1)) = 12.56 +/- 1.96 and E-A = 41.90 +/- 6.08 kcal/mol for supercritical decomposition. Subcritical and supercritical Arrhenius parameters for individual pathways are proposed. The variance in rate parameters is likely due to changing thermophysical properties of water across the critical point. There is strong evidence for a surface catalyzed free-radical mechanism responsible for rapid decomposition above the critical point, facilitated by low density at supercritical conditions. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.