Hydrolysis rate constants of glycylglycine to glycine and equilibrium constants between glycylglycine and diketopiperazine were determined simultaneously in situ using an FT-IR spectroscopy flow reactor at 310-330 degreesC and 275 bar. The hydrolysis of the peptide bond in neutral hydrothermal solution follows the first-order (or pseudo-first-order) rate law. The cyclization of dipeptides is very prominent at high temperatures. Pressure affects the hydrolysis rate and the enthalpy of cyclization of the dipeptide. Specifically, the activation energy for hydrolysis of the dipeptide is reduced by about 50 kJ/mol and the standard enthalpy change of cyclization of the dipeptide is increased by about 50 kJ/mol when pressure is increased from a steam pressure of 1-16 bar to a solution pressure of 275 bar. The data analysis method used in this work is general and could be applied to other dipeptides provided the decomposition rate of the free amino acid product of hydrolysis is monitored.