The rate constants and Arrhenius parameters for the reaction of CO in H2O were determined at 230-270 degreesC and 27.4 MPa by the use of a titanium flow reactor with real-time detection by infrared spectroscopy through sapphire windows. These rate measurements appear to be the first below the critical temperature of water. The zeroth-order kinetics model produced an Arrhenius activation energy of 32 +/- 3 kcal/mol, which is in the range of previously reported values at higher temperatures, but the preexponential factor [ln(A, mol kg(-1) s(-1))] of 20.5 is much larger. The higher overall reaction rate is consistent with heterogeneous catalysis by the reactor surfaces considering (1) the zeroth-order kinetics, (2) the high A factor, (3) the activation energy in the range for the water-catalyzed reactions, and (4) the previously determined dependence of the decomposition rate of the putative formic acid intermediate on the metal used to construct the cell. Extremely toxic Ni(CO)(4) was observed to form as a result of extraction of Ni from slightly corroded 316 stainless steel tubes that connected the cell/reactor to the flow control system. Ni(CO)(4) formed under somewhat limited conditions, but its occurrence forewarns of the potential hazard of hydrothermal processing when a high CO concentration might be present in a nickel-containing reaction vessel.