Infrared spectroscopy measurements of the kinetics and decomposition pathways of aqueous urea ((NH2)(2)- CO, 200-300 degrees C, 275 bar) and guanidinium nitrate ([(NH2)(3)C]NO3, 240-300 degrees C, 275 bar) are described. A Pt/Ir alloy flow cell with diamond wafer windows was used, and heat and fluid transport models show that isothermal and plug flow conditions exist. The hydrothermolysis of urea was modeled by the conversion of urea to NH4+ + OCN- followed by hydrolysis to CO2 + 2NH(3). These reactions are a subset of those for hydrothermolysis of guanidinium nitrate. Decomposition of guanidinium nitrate is catalyzed by the formation of NH3. The reaction scheme involves deprotonation of the guanidinium ion by NH3 to produce neutral guanidine, which hydrolyzes to form urea as the rate-determining step. The subsequent hydrothermolysis chemistry follows that of urea. Thus, although the overall decomposition rate of guanidinium nitrate is slower than that of urea, the Arrhenius parameters for the step for formation of CO2 from guanidinium nitrate and urea are similar [E(a) congruent to 66 kJ/mol, In A (s(-1)) congruent to 19]. Only a small difference in these values is incurred by using a 316 stainless steel-sapphire cell in place of the Pt/Ir-diamond cell. Hence, wall effects appear to be small for this reaction.