Potential for organic synthesis in supercritical water (SCW) was proposed in the 1990's. Here we describe the hydrothermal synthesis of low molecular fluorescent organic molecules derived from an amino acid, glycine, using an SCW flow reactor with rapid expansion cooling. Glycine aqueous solution was treated in SCW and then was poured out into the atmosphere through a narrow orifice of the needle valve to be depressurized. The product mixture solution had emission from blue to ultraviolet. HPLC fractions of the products also had fluorescence and included C5H6N2O and its methyl derivative. Theoretical calculation of fluorescence and absorption spectra of the candidate compounds also shows that the most probable compounds are 5- or 6-methylpyrazin-2-one and their 3-methyl derivatives. The fluorescent compounds suggest carbon-carbon bond formation as well as dehydration condensation in SCW. (C) 2006 Elsevier B.V. All rights reserved.