Infrared spectroscopy in conjunction with an optically accessed flow reactor enable real-time concentration measurements to be made on species involved in hydrothermal reactions. The kinetics of the reaction of the aqueous cyanate ion, OCN-, at 110-160 degrees C and 275 bar were determined at pH = 3.94-10.5. These kinetics are useful because OCN- is frequently the precursor to the NH3 and CO2 products seen during the hydrothermolysis of organic amines. Three rate constants were needed to model the cyanate reaction. These rates were then used to model the hydrothermolysis kinetics of the progressively more complex reactions of the isoelectronic cations of semicarbazide (T = 140-170 degrees C) and aminoguanidine (T = 190-260 degrees C) in neutral and acidic solutions. Six internally consistent rate constants were obtained in order to specify the reaction network.